Is it allowed to join a Texas Hold'em table as the Dealer, the Small Blind or the Big Blind?
If it is, are there any rules to avoid players having to pay the blinds twice?
If you're approaching a game already in progress, you've got a few different scenarios:
1- Open seat is one spot after current big blind -- If you sit here, you will be the big blind on the next hand and no one pays double blinds.
2- Open seat is between current small blind and current big blind. -- If you sit here, you can either sit out the next hand, or you can "buy the button" by posting a big blind and tossing a small blind into the pot. Therefore no one pays double blinds.
3- Open seat is between current small blind and current button. -- If you sit here, the button will simply jump over you on the next hand so that no one pays double blinds.
Casinos have different rules about whether or not a new player has to post a big blind before they can get their first hand. At my local casinos, you have to post if you're moving from a different table, but don't have to post if you're new.
In a tournament when a player joins as the big blind they can play straight away. Otherwise I dont think they are allowed to play until after the Dealer button has passed.
Might be slightly different in cash games. Online you get the option to post the BB straight away, not really sure if that counts for all positions though.
More information can be found here:
I very much doubt someone would ever have to pay the blinds twice in a row or anything like that.