'notmuch search thread:<>' lists multiple threads

[Naveen N. Rao]

Greetings--
If I search for threads matching a specific thread-id, I am seeing 
multiple results:

$ notmuch search --output=threads thread:00000000000c4d20
thread:00000000000c4d1e
thread:00000000000c4d20

If I list the messages from both those threads, they do belong to the 
same original mailing list thread. It isn't clear why notmuch is 
assigning different thread IDs. Is that to be expected under some 
scenarios?

Also, it is a bit weird to see multiple threads being listed when 
searching for a specific thread ID. Again, is this something to be 
expected?


- Naveen