ft 857 Feedback problem

This is the setup to use the ISS repeater...The uplink is a static frequency, the downlink changes with doppler...during FD we noticed this problem.
For some reason this UHF freq was the downlink ...we could hear the audio of other contacts...but when we keyed up...just the instant feedback. I tried with CAT cable connected or disconnected...with a dummy load attached to the UHF antenna port or nothing attached. In this test I start with RF at the minimum, the meter is on the 15w scale, so we are TX at less than a watt. I adjust the squelch, RX gain, preamps, narrow FM, no matter what was selected, we got feedback. Both radios operate exactly the same, at least one of the radios has a pair of SSB filters installed. In one test I even hooked a Duplexer into the receive port...no difference.