Show messages the user hasn't seen before

I'm sharing safety tips with my users.

I only want to show the specific user a safety tip he/ she hasn't seen before (but another newer use might not have seen that tip yet).

Does anyone have any ideas on how I can get that done?

I'm currently allocating random numbers to the tips - and with 5000 tips the user should be ok (i.e. not seeing the same tip twice); but it could happen...