ZF-4269: Zend_Captcha_Image should check if a file with the unique name already exists
Zend_Captcha_Image should check if a file with the generated unique name already exists. On systems where there is a high load it could happen that a unique id gets generated twice. I know this is very unlikely but it could happen.
After generation of the id there should be an check if a file with that name already exists and if yes a new id should be generated. I have created a fix that i will append to this issue. If the unique id generation fails three times an exception will be thrown.