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Is there enough double perf premium for a shady person to risk ruining an already valuable #28 in order to sell it as a variety/error?
I've seen stranger things happen. Nothing surprises me anymore. It's not always done for reasons of profit.
Send it in for expertization. I'm not an expert. The reason I felt it was "iffy" is because it appears the vertical distance displacement between the matching holes in the two columns of perforations are inconsistent. Sometimes the left hole is at same level as the right hole, sometimes it is slightly higher/lower. Again, I'm no expert, but in the double perfs that I've seen, the perfs will either perfectly align horizontally, or the vertical displacement between the matching holes will be very consistent. After all, they were suppose to have been made with the same perforating machine.
Normally, I cut/paste the perforations in the scan to do a direct comparison. I can't do a cut/paste test to try to slowly shift one column vertically to try to find a position that matches, because the stamp scan is not oriented to match horizontal/vertical axes. If the double perf is genuine, you should be able to shift the perf up or down to eventually find a perfect match. It is extremely difficult for a forger to make a perfect match.
If you are planning to sell it, that is one stamp you would want to get a certificate.