Duplicate record with same number

I have created a user register API using laravel passport. when I hit the API then it created duplicate records with the same number sometimes.

What am I doing wrong?

Here's my code, if someone can help me.

public function signup(Request $request) {

    $validator = Validator::make($request->all(), [
                'first_name' => 'required',
                'last_name' => 'required',
                'email' => 'required|unique:users,email',
                'phone' => ['required','unique:users', 'min:10', 'max:13'],
    ]);

    if ($validator->fails()) {
        $this->response['message'] = $validator->errors()->first();
        return response()->json($this->response, 400);
    }

$userCheck = User::where('phone', $request['phone'])->first();
    if ($userCheck) {
        $this->response['message'] = "The user already exists with this number";
        return response($this->response, 400);
    }

$user = User::create([
                'first_name' => $request['first_name'],
                'last_name' => $request['last_name'],
                'email' => $request['email'],
                'phone' => $request['phone'],
    ]);

$token = $user->createToken('Api access token')->accessToken;

return (new UserResource($user, $token))->additional([
                'status' => 1,
                'message' => "registerd successfully"
    ]);
}