unified currency: عملة موحدة

one currency, single currency: عملة واحدة

In some contexts, there may not be much difference; in others, the difference may be significant.

As you can see, these are rather literal renditions. Human language is a lot more flexible than that, giving the translator more options, and at times more headache.

Look at your context carefully and examine the intended meaning rather than the words. "Single currency" may have been used in the sense of "unified currency," for example. It would have been helpful to us if you were so generous as to share the context. Good luck.