A Ham buddy of mine and I were trying to setup a UHF repeater in our area due to a lack of them. We had two Motorola CDMs, one a 1250 and one a 1550, and we purchased a $200 or so reject only duplexer to use from a seller on eBay. When we received the duplexer, we hooked it up and the TX radio severely overpowered the RX and therefore the repeater was useless. We reached out to a local radio shop who found out the duplexer tuning was off due to vibration of shipping and he retuned it for us. The issue was still there and wasn't resolved. RX was perfectly fine until the TX radio keyed up and the repeater went dead on the receiving end out in the coverage area. The TX CDM was transmitting between 30-50 watts. If we were to reduce the output power to say 10-15, would it improve anything? Even then, is the output power even the issue? I know many repeaters in my area that are 45-50 watts that are similar setups that have no issue yet ours gives us problems. Did we just flat out purchase the wrong duplexer (as ours is reject only) and we should explore other options?
In the absence of test equipment there is no way to know for certain what went wrong, or what to do about it. The duplexer sounds like the most likely culprit, but the problem could be something else, like inadequate shielding in the receiver.
If you ask around, maybe someone in your area has the necessary test equipment and wouldn't mind helping.
Perhaps consider the following: use a NanoVNA to check out the duplexer: https://youtu.be/GipCVEsiqXc