G. Sylvester wrote:
If there are
two aircraft operating with the same or similar abreviated
callsigns,
the controller is supposed to advise both aircraft and use the full
callsign for each aircraft.

'supposed to' but not required.

Yes it is required. From the ATC handbook, Order 7110.65P :

2-4-9. ABBREVIATED TRANSMISSIONS

Transmissions may be abbreviated as follows:

a. Use the identification prefix and the last 3 digits or letters of
the aircraft identification after communications have been established.
Do not abbreviate similar sounding aircraft identifications or the
identification of an air carrier or other civil aircraft having an FAA
authorized call sign.


AND...

2-4-15. EMPHASIS FOR CLARITY

Emphasize appropriate digits, letters, or similar sounding words to aid
in distinguishing between similar sounding aircraft identifications.
Additionally:

a. Notify each pilot concerned when communicating with aircraft having
similar sounding identifications.

EXAMPLE-
"United Thirty-one United, Miami Center, U.S. Air Thirty-one is also on
this frequency, acknowledge."

John Galban=====N4BQ (PA28-180)