Only every second submit does work

Made my webform on my testsite, everything works... on the new site the webform block works well, but I included the same form on my homepage, on the testsite everything is good, on the new site submitted from the homepage only every second submit works... have no clue why, although I remember having that on the testsite, too, but then at some point not anymore (wish I could remember better). And I remember having read about this problem somewhere, but can't find that site either.

Please help! Is it only a configuration thing? I thought that everything on the two sites were just the same, but maybe I missed something, though I checked a dozen times...

Thanks!