Can there collision if two threads writing on the same serial port?

I am coding in c#. Can there collision if two threads writing on the same serial port?
If yes, how can I handle this? Also I want to give priority to one of the thread.

For eg. If two threads are writing on the serial port, I want to halt the transmission of thread 1 when I write data from thread 2. i.e. thread 2 has priority of writing it to port than thread1.

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