diff --git a/main_test.go b/errors_test.go
similarity index 94%
rename from main_test.go
rename to errors_test.go
index be2aa55..5512f08 100644
--- a/main_test.go
+++ b/errors_test.go
@@ -25,6 +25,7 @@ func TestErrors(t *testing.T) {
 		{"MissingBlockStart.q", errors.MissingBlockStart},
 		{"MissingGroupEnd.q", errors.MissingGroupEnd},
 		{"MissingGroupStart.q", errors.MissingGroupStart},
+		{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "a"}},
 	}
 
 	for _, test := range tests {
diff --git a/src/build/Function.go b/src/build/Function.go
index f303968..251d5e1 100644
--- a/src/build/Function.go
+++ b/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,
 		}
diff --git a/src/errors/VariableAlreadyExists.go b/src/errors/VariableAlreadyExists.go
new file mode 100644
index 0000000..4cad219
--- /dev/null
+++ b/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)
+}
diff --git a/tests/errors/VariableAlreadyExists.q b/tests/errors/VariableAlreadyExists.q
new file mode 100644
index 0000000..e55bbe6
--- /dev/null
+++ b/tests/errors/VariableAlreadyExists.q
@@ -0,0 +1,4 @@
+main() {
+	a := 1
+	a := 2
+}
\ No newline at end of file