Could this also be an issue with the GPL? What happens if I look at GPL source code and then author similar code under a non-GPL license? I wonder if a precedent has been set that could be applied to this Fair Source license.
In that case it could be argued that your similar code is a "derived work", and hence you are violating the copyright of the original.
Whether that original is GPL'd or not doesn't actually matter from a legal point of view; it could be under a different FOSS licence, or proprietary, the result is the same: if you breach the terms, you're in violation. That's the point of copyleft: it's power comes from exactly the same source as that of proprietary licenses.
In the particular case that your "non-GPL" license just so happens to be similar to the GPL, e.g. it's FOSS, possibly copyleft, etc. then the original author might be feeling generous and be willing to come to some arrangement, e.g. by retroactively granting you permission for your derived work, or by negotiating a licence change with you (changing the license of either your code, theirs or both). Note that this would be orthogonal to the legalities; it's a matter of whether the owner is willing to cooperate. This might be difficult if there are many owners though (e.g. imagine trying to negotiate a license change for the Linux kernel!)