Joe, it's recorded as the standard practice when there are two aircraft within a squadron displaying the same individual code letter.
This came about when squadron strength (fighter squadrons) increased from (nominal) 24 to 36 aircraft, with, of course, all letters of the alphabet then being 'used up'. As numerals were already in use as squadron codes, and as individual codes in some units (eg 9th AF), letters had to be allocated, with the bar to identify the second user (or third user) of the letter in question. There are numerous examples shown in photos (and profiles) from various Fighter Groups, particularly in the 8th AF.
The Directives for identification markings also go on to include bomber units, where a '+' or '-' sign can be used to identify the squadron within a group, where perhaps the individual code is only visible on the vertical fin/rudder. The position of the '+' or '-' indicates the particular squadron within the group. For example, the symbol can be above or below the letter, or forward or aft, and so on.
The system was introduced following the adoption, by the 8th and 9th USAAF, of the RAF code system, when, with the larger squadron and group aircraft allocations, it was found necessary to include these additions for ease of identification.
In the RAF, mainly with bomber squadrons, where squadron strength was increased, if it happened that, for whatever reason, there were two aircraft with the same individual code letter, then a small figure '2' was added, after the letter, for example CF- D2, the code of 625 Sqn, operating Lancasters.
Other RAF solutions were to use '?' and '!' on the relatively rare instances of 'running out of letters'.