It's not a good practice, but really depends on the caliber, and the die used for sizing. The Lee Carbide Factory Crimp Die for pistol calibers does resize the case to a certain degree, but the sizing ring is larger than the case sizing die. If the loaded round was run into the case sizing die after loading, the bullet would be swaged to a smaller diameter, among other things.
For rifle calibers, it's not a good idea at all. For one thing, the neck portion of the die sizes the neck down to a smaller diameter than the bullet by several thousandths. Then the expander button resizes the neck for the proper tension on the bullet. If you were to run a loaded round into the sizing die again, it will size both the bullet and the neck to a smaller diameter, not to mention the force necessary to drive the loaded round into the die (with the decapper removed, of course). This would make the neck of the case thinner and undersized, and the bullet would no longer fit the bore, providing your press is strong enough to perform this function.
There are other reasons this isn't a good practice, but these are the ones that stand out.
Hope this helps.
Fred