There's an incident from not long ago where an RFDS aircraft saw two trainers on final close to each other. He tried to warn them of their proximity only to have the lower one go round and promptly chop the tail off the higher one, which plummeted into the ground inverted. Luckily they both survived the accident. But it goes to show that you have to be very careful how you word things on the radio as the result can go opposite to what you expect. I've also witnessed a situation at Moorabbin where two warriors ended up on top of each other on final at night, the lower aircraft radioed "There's an aircraft above me" the tower calmly called for the higher aircraft to go-round (by callsign), and the lower one to continue. In short I never play controller with other aircraft, just give them my intentions and if they seem confused ask if the can remain clear or something, but never tell them what to do. If in doubt I make way for them to do their thing first.