Landing

How when i am on a final approach and i have been cleared to land how come the ATC lets another plane taxi onto the runway i am landing on when i am like 100 feet away from the runway?
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How when i am on a final approach and i have been cleared to land how come the ATC lets another plane taxi onto the runway i am landing on when i am like 100 feet away from the runway?
I've been cleared for take-off and a second or two later had an incoming cleared for landing but the incoming was 5 or 6 miles out not 100 feet. Maybe a glitch? Does this happen often? And if so, does it happen at several airports or just one?