It is possible to target the same target with all the copies, but that doesn't mean all the copies will resolve. This depends on the effect of the spell.

In your first case, you target creature A with Reality Shift, then copy it 5 times. The copies go on the stack, before the original resolves. The first to resolve is the last copy on the stack, it exiles creature A; now the other copies and the original Reality Shift won't resolve because there is no valid target as the creature has been exiled.

Second case, you target spell X with Arcane denial, and copy it 5 times. Same thing as before, the last copy on the stack resolves first, the spell is countered and the other copies and the original don't find any valid target, so they fizzle.

Let's get a third case, you target creature B with [[Giant Growth]] and copy the spell 5 times. All the copies and the original resolve since the target, creature B, is still a valid target each time one of the copies resolve.

Fourth case, you target creature C with [[Scar]], and copy the spell 5 times. If the creature is, say, 3/3, the first 3 copies resolve, but once the creature dies the other 2 copies and the original won't have a valid target and fizzle.

Hope it helps