Summer catalogs seldom contain enough information. Determine whether the school is on the semester system or the quarter system. If it's on the quarter system, you'll need to take two courses there to get one credit at Duke. For summer courses you should determine how many days the class meets and how long each period is. There must be 35 contact hours before Duke will consider transfer credit.
Some schools offer different versions of the same course, especially for introductory calculus.
For example, for a course to eligible for transfer as Duke's Math 32, in addition to meeting other requirements, it should have as a prerequisite a calculus course equivalent to Duke's Math 31 and must also serve as a prerequisite for higher-level math courses, including multivariable calculus. There are usually a few specific topics that are strong indicators of comparability with Duke courses. We make no attempt to provide a complete list of such topics, but a few are indicated below by course. If the course you are considering doesn't include these topices, then it will probably not be transferable. Green's Theorem, Stokes' Theorem.