I get what you are saying, but you should try Chimera Linux.

Jerboa, if I tried posting a duplicate link into a community, the client will accept it, without even showing a warning.

Sadly, it does not show in Jerboa for Android.

Nope.

I am talking about posts in the same community.

I mean if the same community have the same link in 2 posts, the second one should be detected and deleted.

In that case the second poster should know that this is a deplucate link so they would not post it.

Duckduckgo search did not bring anything valuable.