Even easier to call this bug this way and _very_ easy to replicate under any conditions. Arsenal is the culprit here.
Create a new backpack using this simple config;

 class CfgVehicles
{
	class B_AssaultPack_rgr;
	class bamse_new_bag: B_AssaultPack_rgr
	{
		scope = 2;
		displayName = "Bamse Backpack";
		class TransportItems
		{
			class _xx_30Rnd_65x39_caseless_mag
			{
				name = "30Rnd_65x39_caseless_mag";
				count = 5;
			};
		};
	};
};

Enter arsenal, remove ALL equipment.
Select newly created backpack.
Exit Arsenal and enter it again.
Rinse and repeat, each time you enter Arsenal items will be added again, and again, and again.
You can even click the backpack while already wearing it and the items will start duplicating.

I do get the thought behind how Arsenal acts though, in most cases you want to retain what's in your backpack when switching between to backpacks, like say a green kitbag and a black ... but in no cases should it reapply what's in TransportItems unless going from empty to the backpack in question :)

Video made with the config above, running 1.64.138732 and no other mods: https://youtu.be/EXCXIziOovM

Still an issue. Arsenal even replaces your backpack if you don't change anything on it. Deleting all variables that might be stored on a Backpack. Breaking TFAR without a special workaround.