It's called proliferation - some sort of injury (cold, physical injury) is the usual cause if its only one flower. You may still get fruits from each flower though, be interesting to see how it develops.

Many thanks Bamboo, we will see what they turn out like.
bobj

i had a load of those flowers form on my tom plants last year. They still produced toms. I put it down to the cold summer, this year I only open my polyt door if it is warm enough.

My toms are odd this year i,m getting leaves at the end of the trusses. Then more flower buds ......weird.....

Thanks to all, for the info.