SourceOP has a post about this as well, pretty interesting:
http://www.sourceop.com/modules.php?name=Forums&file=viewtopic&t=93916
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From a TF2 raffler (don't want to put a direct link in-case it is considered advertising):
"Just trying to give you folks a heads up. The following items have been recently duplicated so PLEASE inspect your trades thoroughly. Valve is likely aware of the issue and will probably take action.
Max's head 7 LVL (Original ID: 251524473), Unusual Troublemaker's Tossle Cap Burning (Original ID: 297958481), Unusual Troublemaker's Tossle Cap Purple Energy (Original ID: 418195150), Unusual Chieftain's Challenge (Original ID: 449365543), Unusual Hermes Flaming Lantern (Original ID: 563825193), Unusual Sergeant's Drill Hat Blizzardy Storm (Original ID: 540952507), Unusual Carouser's Capotain Cloudy Moon (Original ID: 549692230), Unusual Hound Dog Scorching flames (Original ID: 278650093), Unusual Tyrant's Helm Scorching flames (Original ID: 228664397),
Want solid proof? Here's a sample of a duped lvl 7 max head.
list of known duplicated items w/ original ID
Most active SPUF thread on this
SourceOP discussion"
Also some links from Reddit Link 1 Link 2
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