I think Ive got it! :)

Changed this file:
https://github.com/RAOF/xf86-video-ati/blob/master/src/radeon_dri2.c

Line 1404 from:
        AddCallback(&ClientStateCallback, radeon_dri2_client_state_changed, 0);

Replaced by:
        if (pScrn->scrnIndex == 0) {
        AddCallback(&ClientStateCallback, radeon_dri2_client_state_changed, 0);
        }

The callback is called only once, even if multiple screens! And the error is
gone!
Please, can someone check if it need to be repeated in other parts as well?
If correct, can someone commit it?

Thank you very much! :)
Weber Kai