chore: add more error test cases for duplicate identifiers

src/error_tests.zig

@@ -14,7 +14,7 @@ test "compiler: unknown global variable" {
     );
 }
 
-test "compiler: global variable shadowing" {
+test "compiler: duplicate variable declarations" {
     try testEqual(
         \\VAR a = 0
         \\VAR b = 2
@@ -28,6 +28,33 @@ test "compiler: global variable shadowing" {
     );
 }
 
+test "compiler: duplicate knot declarations" {
+    try testEqual(
+        \\== a
+        \\== a
+    ,
+        \\<STDIN>:2:1: error: duplicate identifier
+        \\2    | == a
+        \\     | ^
+        \\
+        ,
+    );
+}
+
+test "compiler: duplicate mixed declarations" {
+    try testEqual(
+        \\VAR a = 1
+        \\== a
+        \\Hello, world!
+    ,
+        \\<STDIN>:1:1: error: duplicate identifier
+        \\1    | VAR a = 1
+        \\     | ^
+        \\
+        ,
+    );
+}
+
 test "compiler: invalid divert target" {
     try testEqual(
         \\-> foo