Is it safe to freeze soup that uses previously frozen stock?
Soup (and any food) is safe to freeze and re-use as long as you keep in mind these things:


The time the food has been 'out' does not reset when you freeze it. You are putting any harmful microbes on pause, not killing them. Be aware of the total time since your food has been cooked both before and after freezing. Remember that putting something in the freezer does not result in 'instant' freezing unless you have Carbon freezing equipment.
Soup is often fully re-heated to boiling. This does reset the clock on safety, and is one thing that makes soup easier to freeze and re-use than other foods.
Stock also often changes less with re-cooking than something like meat or vegetables, so you damage the end result less (if at all) with frozen soup than with other re-frozen ingredients.