I always thought this character was interesting - BECAUSE of the fact that she was rarely used (she doesn't even appear in the cartoon series). Hopefully, she finds her way into the MOTUC series.
I consider Double Trouble to be the Zodac of the original She-Ra toyline. You see, Zodac was labeled as the "Evil Cosmic Enforcer" when he was first released, but his character would evolve over the years to be Neutral to Heroic - never evil. So the evil interpretation of the character was short-lived. Speculation on Zodac's allegiance-shifting points to Mattel wanting to balance out the number of evil figures to good ones (see the Wikipedia bio http://en.wikipedia.org/wiki/Zodak). Though Double Trouble was labeled as good in the toy line, I consider her to be evil (her name and role as a spy help, as well as her original prototype color scheme) - Catra needs all the help she can get.
Here's my fleshed out bio for Double Trouble
(influenced by her Wikipedia entry):
Double Trouble was born as the princess of the Kingdom of Green Glade. She is cousin to Glimmer, and similar to Glimmer, was born with magical abilities. Double has the ability to make a magical duplicate of herself. Before she explored the extent of her own powers, she witnessed her cousin Glimmer's growing abilities and became jealous. She sought out a way to increase her abilities. She ventured to Mystacor for the guidance of Castaspella, only to be told to "be patient". She was soon approached by dark Horde magicians, who promised her exactly what she sought.
Through Horde magic, Double's powers were augmented, allowing her to cast powerful mirror-like illusions, and change her appearance to look like anything she could think of. There, however was a side affect: Double could still split herself into two entities, but her personality was split between the two. One entity polarized into the inherently good-natured persona (Green Double Trouble with blonde hair), the other was inherently evil (Blue and Purple Double Trouble with black hair - prototype colors). Because of the Horde magic, the evil Double Trouble was the more powerful portion, and could force the good portion to merge into one at will.
Double Trouble is most powerful when merged into one entity. With the evil portion in command, Double willing serves the Horde. But if the evil portion experiences physical trauma, or becomes incapacitated, the good portion can "split" off and escape (and work with the Rebellion), or take control of the one body. Both portions of Double's personality are eternally struggling to control the one form - which is when they are at their most powerful.