Mmmm…, this look like the typic “difficulty sizing” issue : you are sending multiple solutions for an “already accepted share” (because the proxy act like an unique miner and all your workers mine the same “share”). So the pool tag the other works like “duplicate shares” and reject them…
So basically this is a “pool issue” : the pool is not able to scale the work difficulty proportionally to your aggregated hashrate… You can perhaps use a particular port (harder / ajustable hashrate) or set it yourself (sometimes through the client ‘password’ param, like flypool.org)
What is the pool you use ?