Why have nearly identical aircraft?

Why do many airforces purchase two completely different aircraft at about the same time and yet use them for exactly the same role?

An example of this would be the AH-64 Apache and the AH-1 Cobra, two attack helecopters serving exactly the same role often side by side. The cost of having to have two completely different sets of almost everything (men and equipment) and running two different training programmes for must surely cancel any benefits gained by having two different helicopters.

In the RAF why are Jaguars and Tornados used for the same role of ground attack? I thought when the Tornado was developed initially as the MRCA in the late 70s it was to supersede the Buccaneer, Jaguar and Vulcan in RAF service yet in 1991 during Gulf War 1 the Tornado, Buccaneer and Jaguar all flew sometimes together.