Generally, subsequent calls override the property if you set a new value. With user IDs specifically, the platform operates a little differently. Once a user ID is set the same one should be set for subsequent events. You should not be setting individual user IDs for users because, ultimately, you will be creating new unique users. If you decide to populate the user IDs yourself, it should stick, but this is not a best practice.
For a user property (that refers to specific attributes), once you set those properties in Amplitude, they will persist for the next set of events until you set a new value. And so those new values will override that last set value from the last call.
An Amplitude differentiator is that we keep the historical values of every user property, so that when you query in the past, we are able to return the value the user property had at that point in time.
For helpful definitions of the following terms, refer to our glossary here.
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User property
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Event property
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User ID
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Event ID
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