Some spells like Tail Slap and Tail Sweep work by temporarily granting the caster a "tail of force" with which they can attack. (From what I recall, these are from Dragon Magic or Races of the Dragon.) What happens if the caster in question already has a tail? Does the attack get extra damage from the normal tail, or is it just superseded by being encased in the force tail?
D&D 3.5 does not have general rules to deal with the addition or removal of extremities. This lack is especially glaring when dealing with spells and other effects that grant natural attacks.
In this case, however, the Effect is clear for Tail Slap: "You evoke a dragon's tail". In other words, you create it. Whether you already have a tail is not a factor, the effect of the spell is the creation of a tail.
It is less clear for Tail Sweep, as that spell does not generate an Effect. It could be assumed to work the same, as it also claims to "evoke a dragon's tail", but this is not as well defined.
However, in both cases, the physical appearance of the tail should not be relevant to the spell's mechanics. Should you desire the spell looking like a magical casing around your own tail rather than a new and independent tail, the DM and player could decide just that, without altering the spell's mechanics or balance. This is referred to as "refluffing", and is generally recommended when the description of an ability doesn't match what the player (or DM) desires.
Sadly, no. Because neither spell mentions a bonus for a pre-existing tail, we can assume that whether or not the caster already has a tail is irrelevant. Presumably the force tail simply forms around your current tail.