Hi Alex00,
As mentioned before, MIDI input requires to be deactivated in the connected third party instrument apps, to avoid unwanted double triggering.
The issue is unrelated to Cubasis, and you will experience the same double triggering when running these instruments apps (e.g. iSEM, iMini) on their own.
On a side note:
Apple’s newer Audio Unit format can more or less be seen as the successor of Inter-App Audio.
Many third party developers already have released their instruments and/or effects as Audio Units.
You will not see similar double MIDI trigger problems using AU instruments, plus many more improvements in other areas, compared to IAA.
Hope that helps.
Best,
Lars