I appear to have put some 5w oil into my forks when I should have used 20w.

I know I should but what are the repercussions of not draining the 5w before adding the correct amount of 20w?

you end up with oil that is less than 20 w and therefore less viscous and damping will be affected FWIW some websites argue that not all 20 w is the same anyway for damping.
if you are light i would not be concerned if you are not light it might be an issue

How much have you put in as you can work out what you end up with

Have you added 5W to 20W already in there, or just started with 5W? And what oil is it- damper or lubrication?

Like JY says not all 20W is the same anyway so you could probably find another 20W that performs identically to your 19.5W or whatever it is you’ve made. It might work better for you. Or it might not.