I've had this happen more then three times now. When I have 2 teleport pacts of a site, and I teleport to that site and immediately purchase a replacement, sometimes the new one doesn't stack with the left over one still in my inventory.
Both still work as normal, they just don't stack.
Also when I move both of the teleport pacts from my inventory bag to my packer, they stay seperate. Even when I move them back to my inventory bag they continue to stay seperate.