another duplicate entry

https://en.numista.com/catalogue/pieces856.html
https://en.numista.com/catalogue/pieces19986.html

More importantly, how can we prevent users from creating these kinds of duplicate entries?

EDIT:  While we're on the subject of UK coins, why are the 1990-1997 dates listed for this type:
https://en.numista.com/catalogue/pieces862.html
They should be KM#935a instead.  Did the Rank-Broadley bust really get used eight years ahead of schedule?

We did erase the duplicate.
Well, if the member do not enter a KM... no way to prevent them to create duplicate. (if they enter KM, the computer will let them know this KM already exists)
If we see members that ceate lots of duplicate, we contact them to explain how the search work (often it is the trouble)

Here's another: https://en.numista.com/catalogue/pieces20669.html is a duplicate of https://en.numista.com/catalogue/pieces3849.html