Connect users with tags they create without having duplicate content in mysql database

I want to allow users to enter up to 5 tags tag like a company name, or charity their associated with. When a user clicks on that tag, it will take them to a page with all the users associated with that tag.

example: If somebody enters CBS, it will show all users associated with CBS.

I know I can add a unique index to keep users from entering duplicate content like stated here , but won't that keep users from being able to enter a company they are associated with if it already exist in the database.

I don't wan't duplicates in the database, but do wan't to allow users the choice to pick companies that may already exists in the database submitted from other users.

This database will hold only data submitted from the users.

I am using larval 5

my database looks like this:

Users

id|username|email

Tags

id|tags

usertags

id|userId|tagId

via Chebli Mohamed