diff --git a/exercise-book/src/green-yellow-game.md b/exercise-book/src/green-yellow-game.md
index 5787081..66fa4b8 100644
--- a/exercise-book/src/green-yellow-game.md
+++ b/exercise-book/src/green-yellow-game.md
@@ -111,6 +111,22 @@ fn a_bit_of_everything() {
         "🟩⬜🟨🟨"
     );
 }
+
+#[test]
+fn two_in_guess_one_in_secret() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 3], &[3, 9, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
+
+#[test]
+fn two_in_secret_one_in_guess() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 4], &[3, 3, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
 ```
 
 ## Knowledge
@@ -199,6 +215,8 @@ Let's do this by copying the input, so we can make it mutable, and mark off any
 
 Add `rand = "0.8"` to your Cargo.toml, and make a random number generator with `rand::thread_rng()` (Random Number Generator). You will also have to `use rand::Rng;` to bring the trait into scope.
 
+(A built-in [random number generator](https://github.com/rust-lang/rust/issues/130703) is proposed for the Standard Library but is still nightly only as of October 2024).
+
 Call `your_rng.gen_range()` in a loop.
 
 <details>
diff --git a/exercise-solutions/green-yellow/src/bin/complete.rs b/exercise-solutions/green-yellow/src/bin/complete.rs
index 2782554..5ed411d 100644
--- a/exercise-solutions/green-yellow/src/bin/complete.rs
+++ b/exercise-solutions/green-yellow/src/bin/complete.rs
@@ -4,21 +4,34 @@ use rand::Rng;
 
 fn calc_green_and_yellow(guess: &[u8; 4], secret: &[u8; 4]) -> String {
     let mut result = ["⬜"; 4];
-    let mut guess = guess.clone();
-    let mut secret = secret.clone();
+    let mut secret_handled = [false; 4];
 
     for i in 0..guess.len() {
         if guess[i] == secret[i] {
+            // that's a match
             result[i] = "🟩";
-            secret[i] = 0;
-            guess[i] = 0;
+            // don't match this secret digit again
+            secret_handled[i] = true;
         }
     }
 
-    for i in 0..guess.len() {
-        for j in 0..secret.len() {
-            if guess[i] == secret[j] && secret[j] != 0 && guess[i] != 0 {
-                result[i] = "🟨";
+    'guess: for g_idx in 0..guess.len() {
+        // only process guess digits we haven't already dealt with
+        if result[g_idx] == "🟩" {
+            continue;
+        }
+        for s_idx in 0..secret.len() {
+            // only process secret digits we haven't already dealt with
+            if secret_handled[s_idx] {
+                continue;
+            }
+            if guess[g_idx] == secret[s_idx] {
+                // put a yellow block in for this guess
+                result[g_idx] = "🟨";
+                // never match this secret digit again
+                secret_handled[s_idx] = true;
+                // stop comparing this guessed digit to any other secret digits
+                continue 'guess;
             }
         }
     }
@@ -147,3 +160,19 @@ fn a_bit_of_everything() {
         "🟩⬜🟨🟨"
     );
 }
+
+#[test]
+fn two_in_guess_one_in_secret() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 3], &[3, 9, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
+
+#[test]
+fn two_in_secret_one_in_guess() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 4], &[3, 3, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
diff --git a/exercise-solutions/green-yellow/src/bin/step4.rs b/exercise-solutions/green-yellow/src/bin/step4.rs
index 66af63c..158eac0 100644
--- a/exercise-solutions/green-yellow/src/bin/step4.rs
+++ b/exercise-solutions/green-yellow/src/bin/step4.rs
@@ -2,21 +2,34 @@
 
 fn calc_green_and_yellow(guess: &[u8; 4], secret: &[u8; 4]) -> String {
     let mut result = ["⬜"; 4];
-    let mut guess = *guess;
-    let mut secret = *secret;
+    let mut secret_handled = [false; 4];
 
     for i in 0..guess.len() {
         if guess[i] == secret[i] {
+            // that's a match
             result[i] = "🟩";
-            secret[i] = 0;
-            guess[i] = 0;
+            // don't match this secret digit again
+            secret_handled[i] = true;
         }
     }
 
-    for i in 0..guess.len() {
-        for j in 0..secret.len() {
-            if guess[i] == secret[j] && secret[j] != 0 && guess[i] != 0 {
-                result[i] = "🟨";
+    'guess: for g_idx in 0..guess.len() {
+        // only process guess digits we haven't already dealt with
+        if result[g_idx] == "🟩" {
+            continue;
+        }
+        for s_idx in 0..secret.len() {
+            // only process secret digits we haven't already dealt with
+            if secret_handled[s_idx] {
+                continue;
+            }
+            if guess[g_idx] == secret[s_idx] {
+                // put a yellow block in for this guess
+                result[g_idx] = "🟨";
+                // never match this secret digit again
+                secret_handled[s_idx] = true;
+                // stop comparing this guessed digit to any other secret digits
+                continue 'guess;
             }
         }
     }
@@ -99,3 +112,19 @@ fn a_bit_of_everything() {
         "🟩⬜🟨🟨"
     );
 }
+
+#[test]
+fn two_in_guess_one_in_secret() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 3], &[3, 9, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
+
+#[test]
+fn two_in_secret_one_in_guess() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 4], &[3, 3, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
diff --git a/exercise-solutions/green-yellow/src/bin/step5.rs b/exercise-solutions/green-yellow/src/bin/step5.rs
index 6d345df..57d8d7f 100644
--- a/exercise-solutions/green-yellow/src/bin/step5.rs
+++ b/exercise-solutions/green-yellow/src/bin/step5.rs
@@ -4,21 +4,34 @@ use rand::Rng;
 
 fn calc_green_and_yellow(guess: &[u8; 4], secret: &[u8; 4]) -> String {
     let mut result = ["⬜"; 4];
-    let mut guess = *guess;
-    let mut secret = *secret;
+    let mut secret_handled = [false; 4];
 
     for i in 0..guess.len() {
         if guess[i] == secret[i] {
+            // that's a match
             result[i] = "🟩";
-            secret[i] = 0;
-            guess[i] = 0;
+            // don't match this secret digit again
+            secret_handled[i] = true;
         }
     }
 
-    for i in 0..guess.len() {
-        for j in 0..secret.len() {
-            if guess[i] == secret[j] && secret[j] != 0 && guess[i] != 0 {
-                result[i] = "🟨";
+    'guess: for g_idx in 0..guess.len() {
+        // only process guess digits we haven't already dealt with
+        if result[g_idx] == "🟩" {
+            continue;
+        }
+        for s_idx in 0..secret.len() {
+            // only process secret digits we haven't already dealt with
+            if secret_handled[s_idx] {
+                continue;
+            }
+            if guess[g_idx] == secret[s_idx] {
+                // put a yellow block in for this guess
+                result[g_idx] = "🟨";
+                // never match this secret digit again
+                secret_handled[s_idx] = true;
+                // stop comparing this guessed digit to any other secret digits
+                continue 'guess;
             }
         }
     }
@@ -108,3 +121,19 @@ fn a_bit_of_everything() {
         "🟩⬜🟨🟨"
     );
 }
+
+#[test]
+fn two_in_guess_one_in_secret() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 3], &[3, 9, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
+
+#[test]
+fn two_in_secret_one_in_guess() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 4], &[3, 3, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
diff --git a/exercise-solutions/green-yellow/src/bin/step6.rs b/exercise-solutions/green-yellow/src/bin/step6.rs
index 80fca99..62ce74f 100644
--- a/exercise-solutions/green-yellow/src/bin/step6.rs
+++ b/exercise-solutions/green-yellow/src/bin/step6.rs
@@ -5,28 +5,40 @@ use rand::Rng;
 #[allow(unused)]
 fn calc_green_and_yellow(guess: &[u8; 4], secret: &[u8; 4]) -> String {
     let mut result = ["⬜"; 4];
-    let mut guess = *guess;
-    let mut secret = *secret;
+    let mut secret_handled = [false; 4];
 
     for i in 0..guess.len() {
         if guess[i] == secret[i] {
+            // that's a match
             result[i] = "🟩";
-            secret[i] = 0;
-            guess[i] = 0;
+            // don't match this secret digit again
+            secret_handled[i] = true;
         }
     }
 
-    for i in 0..guess.len() {
-        for j in 0..secret.len() {
-            if guess[i] == secret[j] && secret[j] != 0 && guess[i] != 0 {
-                result[i] = "🟨";
+    'guess: for g_idx in 0..guess.len() {
+        // only process guess digits we haven't already dealt with
+        if result[g_idx] == "🟩" {
+            continue;
+        }
+        for s_idx in 0..secret.len() {
+            // only process secret digits we haven't already dealt with
+            if secret_handled[s_idx] {
+                continue;
+            }
+            if guess[g_idx] == secret[s_idx] {
+                // put a yellow block in for this guess
+                result[g_idx] = "🟨";
+                // never match this secret digit again
+                secret_handled[s_idx] = true;
+                // stop comparing this guessed digit to any other secret digits
+                continue 'guess;
             }
         }
     }
 
     result.join("")
 }
-
 fn main() {
     let mut rng = rand::thread_rng();
     let stdin = std::io::stdin();
@@ -143,3 +155,19 @@ fn a_bit_of_everything() {
         "🟩⬜🟨🟨"
     );
 }
+
+#[test]
+fn two_in_guess_one_in_secret() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 3], &[3, 9, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}
+
+#[test]
+fn two_in_secret_one_in_guess() {
+    assert_eq!(
+        &calc_green_and_yellow(&[1, 2, 3, 4], &[3, 3, 9, 9]),
+        "⬜⬜🟨⬜"
+    );
+}