Routing audio from external hardware synth through First Gen Fousrite 18i8 issue.

Does your interface have a software mixer? I had one with my Focusrite Saphire years ago, as it might have the fader for those inputs at unity so you're hearing everything twice (as in via Focusrite's mixer/input monitoring) and then through Logic.

If you have a console/mixer program, mute those channels or switch off Software Monitoring and to see if the doubling audio goes away so that you're not hearing the synth through Logic and the inputs. Alternatively if you want to hear the dry signal then set up a dummy audio channel in Logic (inputs are the synth but output is set to No Output) and record that way.

Alternatively if your synth also is connected via MIDI to your iMac - set up a new software instrument, choose External as the instrument, select which Midi interface is being used and the audio inputs it's connected to. You can then record midi which the synth can play back and then record the audio (on a track with the input channels connected to the synth) whilst moving filters/ADSR etc which can be fun whilst the midi plays the notes. If you mute the channel and have the audio coming through Focusrite's mixer then you'll only hear the source sound, or mute the Focusrite mixer channels and hear it play via the External software instrument channel.