That usually happens when you already have an app with the same app ID installed (even if it is in a different profile).
Do you have Sandboxed Google Play installed on one of your profiles? Then that's exactly why you can't install Gcam service provider that tries to spoof one of the official app IDs.
Google Camera can be used with the sandboxed Google Play compatibility layer and can take full advantage of the available cameras and image processing hardware as it can on the stock OS. It currently only depends on GSF and can be used without Play services or the Play Store.
Hope this clarifies things