Layouts downloaded via the Community Layouts browser cannot be uploaded again.
This is in order to prevent slight variations of the same layout to be shared by multiple users thus 'muddying' the layout collection with copies..
You can just open the local version of your layout (that you've originally shared) and share it again;
When you have a similar layout uploaded earlier the 'Update' option will be shown in the Share/Upload window enabling you to update the existing layout in the cloud instead of uploading a completely new one.
If the problem persists please drop us a short email and we'll look into it right after our Easter break!