Pon en el evento OnKeyPress del edit
Código Delphi
[-]
if ( StrScan('0123456789.'+chr(7)+chr(8), Key) = nil ) then Key := #0;
if key = '.' then key := ',';
for i := 1 to length(EDos.Text) do
if ( copy(EDos.Text,i,1) = ',' ) and not ( StrScan(',', Key) = nil ) then Key := #0;