Depends on the flooring. What do you have now and what do you want to put down? How many layers do you currently have?
What is the old flooring and what is the new flooring. Diffeerent flooring different ways.
It will depend on what type of flooring each one. For example, tile should have cement board put down then tile. Hardwood flooring needs a moisture/vapor barrier underneath such as roll tar paper that goes between roofing shingles and decking. Most other type floors would work with putting luane down then the new floor. In a few cases you can put the new floor over the old, but those are in the minority.
Need more information please. But it is,usually better to remove the old floor.
If it is click flooring then there is no problem providing the old floor is level and in good condition. Watch for the transition between rooms but there are special strips to match your flooring to take care of this.
More often than not, removing the old floor first is best as going over it can ruin both floors, costing you way more. Some laminates can be installed over existing but I still would not advise it.