Read, check, write is three steps and two clients fit between them: make it one command.

Redis is atomic per command, not per sequence. If two requests both read 1 before either writes, both think they got the last seat. Fold the logic into one server-side operation: a DECR, or a short Lua script that refuses to go below zero. Branching on a value you just read cannot be done with queued commands alone.

Read it once, then a question tests whether the trick stuck.