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The Magic 2.0 series by Scott Meyer has this situation with a core character (so it's not a passing situation). The narrator and the characters identify the two as Brit the Elder and Brit the Youn...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/46537 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The _Magic 2.0_ series by Scott Meyer has this situation with a core character (so it's not a passing situation). The narrator _and the characters_ identify the two as Brit the Elder and Brit the Younger. When more time-travel shenanigans happen, we also encounter Brit the Even Elder and Brit the Much Younger, which doesn't seem sustainable but these are shorter scenes. An alternative to these kinds of sequential references is "place of origin" references, if that's meaningful to you. If you have a time-traveler from way in the future visiting a modern-day character, they might be New York Bob and Alpha Centauri Bob or, more ominously, Last-Days Bob and Post-Cataclysm Bob. Any reference that readers and all involved characters will (come to) understand and that is _durable_ works.