Some companies implement then un-implement this rule as franchises change, and the only indication is in the application progress.
"Do you have any other live applications for traincrew with this company?" often gets asked on the application start page. Others demand no further applications from candidates for the next six months - but unless you see it in black and white, you should assume that no such rule applies at that operator.
The only blanket rule (used to be) on the operational staff assessments (aka OPC, Group Bourdon, Scaaat, Khaaan etc)... these are generally maximum two goes in a lifetime, minimum six months apart - although I've seen that rule waived a lot recently - especially as by law, candidate data must be deleted after a certain time.