How do we deal with multiple values for a key?

We all know this situation: you need to add a telephone number to a node and add the line:
phone=00311198765432
Then you find out that there is a second phone number for that node, but you can’t add a second phone= tag because OSM doesn’t allow that.
The general question is: how do I tag multiple values for one key?
Let’s investigate how mappers have solved that problem sofar. The screenshots all are made with OpenPoiMap.

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[1]
This example is the Eiffel tower in Paris for which four architects worked together, but only one gave his name to the final product!
In the source the names are separated with semicolons:
Stephen Sauvestre;Gustave Eiffel;Maurice Koechlin;Émile Nouguier

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[2] One piece of art created by 5 artists (somewhere in Seattle).
Create a new key with a sequential number attached to it for every member of this group. In this specific case I would have started numbering with artist_number_2 because the first one is already in artist_name. Even better, I would have started with artist_name_1 and used it for Andrew Keating and would have omitted artist_name altogether.

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[3] This example is to show how to map multiple sets of related tags to one node. In this case we have man_made=mast which has attached to it 4 (mobile phone) antennas at different heigth and each working with a different technology.
An underscore could have been used as in the previous example, but there seems to be a tendency to tag situations like this with a key:N notation, where N is running over the natural numbers.

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[4] Here we see a combination of both methods. Adding a number (with underscore) at the end of the key to count them or adding the number after the colon. Again, in the case of the fuel I would have started with fuel:diesel:1 for the Biosolar.

Any different opinions on this subject?