706.9e. Some replacement effects that generate copy effects include an exception that's an additional effect rather than a modification of the affected object's characteristics. If another copy effect is applied to that object after applying the copy effect with that exception, the exception's effect doesn't happen.

Example: Altered Ego reads, "You may have Altered Ego enter the battlefield as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it." You choose for it to enter the battlefield as a copy of Clone, which reads "You may have Clone enter the battlefield as a copy of any creature on the battlefield," for which no creature was chosen as it entered the battlefield. If you then choose a creature to copy as you apply the replacement effect Altered Ego gains by copying Clone, Altered Ego's replacement effect won't cause it to enter the battlefield with any +1/+1 counters on it.

copying something else wipes out the other bonuses of the first copy effect

4/15/2013 If you choose to have Progenitor Mimic enter the battlefield as a copy of a creature, the triggered ability it gains will become part of its copiable values. For example, suppose Progenitor Mimic enters the battlefield as a copy of Runeclaw Bear, a 2/2 green Bear creature with mana cost 1Green. The resulting object is a 2/2 green Bear creature named Runeclaw Bear with mana cost 1Green and with “At the beginning of your upkeep, if this creature isn’t a token, put a token onto the battlefield that’s a copy of this creature.” If another Progenitor Mimic enters the battlefield as a copy of that creature, it will be a Runeclaw Bear with two instances of the triggered ability.