The destinations the probes go to are drastically different, and generally the things you want to study aren't identical. What's more is that the mission plans for probes need to be extremely detailed, which impacts the design of the actual probes. The extreme level of detail necessary for a mission is also one of the reasons probes aren't launched on short timeframes.
However, they are absolutely used for more than one purpose, and this is part of the reason the mission plans are so detailed. We generally try to get as much usable science out of what we send. For example, the mission for Cassini Huygens[1] included Venus, Jupiter, and obviously Saturn and its moons. Some of the other easy examples... Voyager[2] I and II, Spirit and Opportunity[3].
However, they are absolutely used for more than one purpose, and this is part of the reason the mission plans are so detailed. We generally try to get as much usable science out of what we send. For example, the mission for Cassini Huygens[1] included Venus, Jupiter, and obviously Saturn and its moons. Some of the other easy examples... Voyager[2] I and II, Spirit and Opportunity[3].
[1] https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens
[2] https://en.wikipedia.org/wiki/Voyager_program
[3] https://en.wikipedia.org/wiki/Mars_Exploration_Rover