The plugin of the message which registered first will trigger first.
E.g.
Update message of Contact is registered for Plugin A
Then, Update message of Contact is registered for Plugin B with the same pipeline stage and execution order.
When the Contact is updated, Plugin A will be triggered first, followed by plugin B.
Now, Unregister the Update message of Contact for Plugin A
Then, register again for the Update message of Contact for Plugin A
When the Contact is updated, Plugin B will be triggered first in this time.