Try keying the repeater with the LEAST amount of power.
At that point if you can't "hold" the repeater, increase your power level until you can.
THIS will be the amount of desense (IF you have it.)
Preamps will only make things worse.
In your location, I would reccomend installing a preselector filter like the DCI 145-2H for $169.95 It can be installed AHEAD of the duplexer and will dramatically attenuate out of band signals from reaching the transmitter as well as the receiver.
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