No, iPhoto does not delete files after a certain period of time
No, iPhoto does not delete files with the same name, it handles that issue effortlessly.
Usual cause: a damaged Library.
Try these in order - from best option on down...
1. Do you have an up-to-date back up? If so, try copy the library6.iphoto file from the back up to the iPhoto Library (Right Click -> Show Package Contents) allowing it to overwrite the damaged file.
2. Download iPhoto Library Manager and use its rebuild function. This will create a new library based on data in the albumdata.xml file. Not everything will be brought over - no slideshows, books or calendars, for instance - but it should get all your albums and keywords back. Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
3. If neither of these work then you'll need to create and populate a new library.
To create and populate a new iPhoto 08 library:
Note this will give you a working library with the same Events and pictures as before, however, you will lose your albums, keywords, modified versions, books, calendars etc.
In the iPhoto Preferences -> Events Uncheck the box at 'Imported Items from the Finder'
Move the iPhoto Library to the desktop Launch iPhoto. It will ask if you wish to create a new Library. Say Yes. Go into the iPhoto Library (Right Click -> Show Package Contents) on your desktop and find the Originals folder. From the Originals folder drag the individual Event Folders to the iPhoto Window and it will recreate them in the new library. When you're sure all is well you can delete the iPhoto Library on your desktop.
In the future, in addition to your usual back up routine, you might like to make a copy of the library6.iPhoto file whenever you have made changes to the library as protection against database corruption.