When submitting a donation/membership form the timestamp of the last submission must be stored in the session. When the form is submitted again, the timestamp must be checked and when the time difference is lower than a configurable value, the user sees a page with the following text (mail address must be a mailto link):
Sie haben vor sehr kurzer Zeit bereits gespendet. Bitte warten Sie einen Moment.<br/> Wenn Sie Fragen dazu haben, wenden Sie sich bitte an MAIL@EXAMPLE.COM
This "protection" can be easily subverted by disabling cookies, in the future T123125 should be implemented. But for feature parity and some protection we need this.