Lookup possibilites

[Saturn]

I have a table with hundreds of rows with city pairs, lik the table to the left.
The list is unsorted. I use the values referring to a city pair in another sheet, using VLOOKUP.

I will make the list more easy to handle with showing a city pair only once, regardless of the direction.
This means that when I register for instance LONNYC, I don't need to register NYCLON, in order to pick up
the (same) value.
Is it possible, and how should the lookup function be constructed ?

Please see the enclosed file.

[humdingaling]

deconstruct the lookup value

in attached example
Cell D21 is the lookup value

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