Ear bones in mammals. One of those reptile lower jaw bones was where the joint with the upper jaw was formed. In some of the transitional fossils, it can be seen that the jaw joint is duplicated. The new joint evolved before the old one was converted to its new function. So this was evolution by duplication of function by two different sets of parts.
Many genes are partially redundant.
There are multiple overlapping pathways. If you use genetic engineering techniques to knock them out, the animal still lives, even though it is missing what was thought to be an important gene. If duplication is a good way of evolving new traits, then lack of duplication may be regarded as a constraint. Therefore, the cascade of changes may be limited to the compartment and doesn't necessarily affect the whole organism. The organelles within a cell restrict certain activities to certain areas- mitochondrion can carry out its special function without interfering much with the rest of the cell Different cell types do the same thing.