You can also make external Id in unique

and

You can write trigger for checking duplicates,You are not getting "Popup",you will get page message

Already a lot of posts are available for duplicate triggers

check below links

https://success.salesforce.com/apex/answers?id=9063A000000DazuQAC

Trying to create a trigger to prevent creation of duplicate accounts

You can write trigger for checking duplicates,You are not getting "Popup",you will get page message

Already a lot of posts are available for duplicate triggers

check below links

https://success.salesforce.com/apex/answers?id=9063A000000DazuQAC

Trying to create a trigger to prevent creation of duplicate accounts