We know of at least 370 BTC returned by people for sure.
124.84280414 BTC by 17 different people onto a general address.
and 245.7032832 BTC by Zephyr onto an individual address.
Nefario assigns different addresses to different people, which is in accord with popular demand.

These amounts looks like being returned as well:
55.7246187 BTC http://blockchain.info/address/14CQiJAiA5o8go8PWc27BjkNA4v2t7QFEt
37.9993912 BTC http://blockchain.info/address/1AcNDLYUwtDFbUhJLKa5DH7Rw9PKpSy2D7

It sums up to about a half of the double payments. Garr255 cited Nefario saying 3/4 of extra coins are back.
The "honesty factor" is not small and is still growing.

Nefario can give issuers all the data without the assets of the ones who haven't returned the coins.