Confirmed, and your screenshot is exactly it — the Item Pouch and Meso Magnet are sitting in the pet window and in the Cash tab at the same time.

The cause: the pet slots were only ever bookkeeping. Fitting an item wrote its name into a slot but never actually took the item out of your bag, and taking it off just blanked the slot without giving anything back. In the real game these are ordinary equips — putting one on your pet moves it out of the inventory the same way a hat moves out when you wear it.

There are three places that need the same fix, and all three are in the patch: dragging an item onto the pet window, double-clicking it in the bag, and clicking a filled pet slot to take it off (that one has to hand the item back to the Cash tab, and refuse if the tab is full, so nothing can be destroyed).

One thing worth knowing while this is open: because the item never left your bag, nothing was duplicated permanently — you just had one item being drawn twice. After the fix it will show in one place only.

Confirmed, and your screenshot shows it perfectly — the Meso Magnet is fitted in the VESO MAG cell AND still sitting in the Cash tab, and the same for the Item Pouch. Fitting a pet item only ever wrote into the pet window; nothing took it out of your bag, and taking one back off never returned it either (the code's own comment said "the caller owns the bag side", and no caller did). That's now fixed on both sides: a pet item leaves your bag when it goes into a pet slot and comes back to your bag when you take it out. If your bag happens to be full when you take one off, it stays fitted rather than vanishing. Two notes: this covers both ways of fitting one (dragging onto the pet window and double-clicking it), and any duplicate you already have from before the fix will still be in your bag — drop or sell the spare.