The Double Trouble series by Michael J. Pellowski is about a pair of twins, Randi and Sandi Daniels.  The two girls look identical but have very different interests and skills.  Because even their parents and younger brother Teddy sometimes have trouble telling the two of them apart when they’re dressed alike, the girls sometimes switch places.  Usually, they swap identities because one of them is needed to do something that the other girl would have trouble doing by herself (like getting a better grade in spelling) or because one of them wants to be two places at once, doing different things.  Of course, most of their schemes to switch places backfire in some way or snowball into an even more ridiculous situation.

In some of the books, the girls are joined by their cousin Mandy, who looks very much like the twins.  The explanation for their resemblance to their cousin is that their fathers were brothers and their mothers were sisters.

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Randi loves sports, but Sandi is more studious.  When the girls’ parents tell Sandi that they’d like her to make more of an effort in gym and Randi that she needs to improve her spelling or get special tutoring, the girls switch places to help each other out.

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When their snobbish cousin Mandy comes for a visit and shows off her acting skills by pretending to be each of her twin cousins (at different times), Randi and Sandi decide that they have to teach her a lesson.

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Randi and Sandi go to visit Mandy, and when she tells them about trying out for a part in a commercial, they consider a scheme that will give Mandy an edge and help her get back at a rival for the part.

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Randi and Sandi are spending the summer at a lake.  Randi enters a fishing contest with a friend, and Sandi is trying to earn a wilderness merit badge.  An accident with a skunk sets off a situation where the girls need to switch places.

A Double Trouble Dream Date, the twins conspire to get roles in a new music video with a teen star, when only Sandi is actually able to sing.

Double Trouble Mystery Mansion, the girls are investigating a haunted house which holds a hidden treasure.  There is a real ghost in the story, but only Sandi has been able to see it.  At first, Randi doesn’t believe her that there even is a ghost, and the girls attempt another switch to see if they can convince the ghost to show himself to Randi, too.