What type of photoeye are you using? Through-beam, diffuse, or retroreflective? Something in the product you're detecting or the way the photoeye is mounted is causing the signal from the eye to go TRUE-FALSE-TRUE-FALSE for a single pass of the product in front of the eye. There could be more TRUE_FALSE transitions that are happenening faster than the PLC can detect.
Put a scope on the signal to see what it looks like. Then adjust your mounting or your gain setting on the eye to get the cleanest signal.
If you can't fix it by these adjustments, then you could create a timer in ladder logic to ensure that a minimum time period elapses between successive counts.