Added more tests

src/build/Function.go

@@ -131,13 +131,21 @@ func (f *Function) CompileInstruction(line token.List) error {
 			return errors.New(errors.MissingAssignValue, f.File, expr.LastChild().Token.After())
 		}
 
-		name := expr.Children[0]
+		name := expr.Children[0].Token.Text()
+		_, exists := f.Variables[name]
+
+		if exists {
+			return errors.New(&errors.VariableAlreadyExists{Name: name}, f.File, expr.Children[0].Token.Position)
+		}
+
 		value := expr.Children[1]
 
+		// All expressions are returned to the memory pool.
+		// To avoid losing variable values, we will remove it from the expression.
 		expr.RemoveChild(value)
 
-		f.Variables[name.Token.Text()] = &Variable{
-			Name:    name.Token.Text(),
+		f.Variables[name] = &Variable{
+			Name:    name,
 			Value:   value,
 			IsConst: true,
 		}

src/errors/VariableAlreadyExists.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// VariableAlreadyExists is used when existing variables are used for new variable declarations.
+type VariableAlreadyExists struct {
+	Name string
+}
+
+// Error generates the string representation.
+func (err *VariableAlreadyExists) Error() string {
+	return fmt.Sprintf("Variable '%s' already exists", err.Name)
+}