[Zope] Refresh-Product

Hi, i have a question about the refresh product. After i installed the product it works fine, for a while. At present there is something strange. After i had refreshed my own product for a few times (without any changes of the .py file) , it appears as a multiple entry in the add-Product list box. If i restart the whole zope, what i would prevented, the multiple entries will reduce to one entry. Has anybody an idea what's going on? Ralph