I know of at least one case where the relevant science community would love to have more identical spacecrafts that are launched about one year apart: Follow ups to the very successfull stereo mission. In 2006 we launched two spacecraft that (compared to other science missions) were basically identical, with the goal of having solar observations not just from earth, but two other vantage points. During the mission it we learned that it is quite helpful to combine Stereo data with data from the Soho satellite that is stationed near earth at the L1 Lagrange point. And while having two or three observations from different directions is a start, having a couple more stereo-like satellite would make things like 3d reconstruction of CME fronts possible.
I strongly suspect this isn't true.
We generally have to make a choice, and new types of science gathering usually wins. For a variety of reasons, some good, some merely political.
But it's a choice, not a fact of life.