Why a second aircraft type can be useful rather than redundant

Were the Sea Wolf and similar programmes unnecessary once Avenger was available?

Not necessarily. A second type adds capacity and production insurance, although the fleet must accept a larger logistics burden.

A single successful aircraft can perform the mission, but relying on it concentrates production and deployment risk. Additional types provide flexibility and reduce the pressure on the leading design. Two aircraft also require separate spares, training, and supply arrangements. Parallel procurement is rational when industrial insurance and extra capacity are worth more than the burden of standardization.