If you pump item into a sorting interface attached to a chest(inventory) and set the priority to low, item will show up in both the chest(inventory) that attaches to a sorting interface and the other chests(inventory) that has higher priorities.

I can't seem to reproduce this, could you go more indepth with your setup (what are you using to move the items, what chests are you using, what kind of filters do you have set, etc)? A screenshot of your setup would be ideal, so I could rebuild it.

Two sorting interfaces attached to two barrels(can be any types of inventory space). Both sorting interfaces set to blank black list. Left one has normal priority. Right one has normal-high priority. Left barrel hooked up to a relocator with a extraction module(can be any types of transferring method). And...

Slightly different from I mentioned above. I was thinking items would show up in both barrels. But it does dupe up.

Ah, this looks like #304 which has been fixed for the upcoming new release.

Thanks for the elaborate screeshots though!