COD MW2 PC Multiplayer something very strange please help ?

sometimes after i finish the match and i make sure i leave the lobby .but i'm still logging into steam just editing my new weapons
and than after a while when i come back to play again i found my self already inside a match that i didn't choose and i'm most of the time i'm at the end of it and probably get killed .

if that mean someone log into the game instead of me at the same time or what that mean ?
i'm new btw
 
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