I agree with Tree Climber if you are hunting on public land then find another spot. I hunt public land and the rule I've always practiced is first come first serve, If somebody else scouts out a spot and puts a stand up before you get out there then find another spot. Under no circumstances would I sit in another hunters stand.
If you are hunting on private land then you need to talk to the landowner and see who else has permission to hunt there. If they haven't given anyone else permission then put your stand wherever you want but if they have then I'd treat it like public land, if they were there first then find another spot.
There will probably be some people who disagree and think that anything goes on public land but courtesy and common sense have to prevail. How would you like it if you invested hours and hours scouting the area, learning the deer patterns, selecting the spot to place you stand, hanging your stand and trimming your shooting lanes, etc, etc... Then you get up and head out to hunt excited as hell to see if all your hard work pays off and when you get to your stand here's somebody else sitting in it. I would ask them not so politely to get out of my stand and if they refused I would tell them "Fine stay in my stand but it's coming down, with or without you in it".
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