In order to make your data more reliable, you can prohibit the generation of duplicates.

Example

I want each contact in my CRM to be unique. Therefore, filling in an already existing contact will cause an error message to appear and my contact will not be created.

Process

Identify the column on which you want to prohibit the generation of duplicates then modify it  :

Check the "Part of the unique key" box.

You can of course assign the option to multiple columns in the same table. This will cause a concatenation of these people in order to create a unique key and thus prevent the creation of duplicates on this key.