Sometimes you can find questions on forums like a:
«I have detected higher ratio of duplicated shares than earlier?» or
«Duplicated shares — what does it mean?»
Essentially, duplicate shares phenomenon are happens when two or more hashing units send to pool the same proofs of work. In fact, pool can accept only one.
Why it happens? Most common cases are mistakenly configured pool or bugs in miner software. So, it could be solved by switching to another pool. If others pools are showing high amount of duplicate shares, than problem is in miner software.
It is quite long story how to fix miner software bugs, cause too many interdependencies are exist there.