I'm directly dealing with Bitcoin RPC and my first step is loading wallet. At first I try to check listwallets to find the current wallet there. If not, trying to load wallet:

Post body = {"method":"loadwallet","params":["aaa34f5c59305e2a13d5438e0ce*****"],"id":"jsonrpc"}
Address = /wallet/aaa34f5c59305e2a13d5438e0ce*****

Sometimes as a result of loading request I get:

{"result":null,"error":{"code":-4,"message":"Wallet file verification failed: Error loading wallet aaa34f5c59305e2a13d5438e0ce*****. Duplicate -wallet filename specified."},"id":"jsonrpc"}

So, this results in error, but after some time (a couple of minutes) listwallets contains my wallet (aaa34f5c59305e2a13d5438e0ce*****). What did happen?

I also noticed that listwallets always contains 50 results. I didn't find in the documentation about this limit but it can be somehow related.