What chances are to have token collision/duplication during token generation upon multi-server php app?

I have a table containing urls that a user can visit:

id PK AUTOINCREMENT
token VARCHAR (8)
user_id_visited INTEGER
visited BOOLEAN

The url is in the form of https://example.com/mypage/^token^ where ^token^ is retrieved from ^token^ field above. What I want to achive it to block a user having user_id_visited revisit…