Do not crash on re-registration

Some language servers such as OmniSharp may (erroneously but not
fatally) re-register the same capability multiple times.
Instead of crashing in such case, just log.

org.eclipse.lsp4e/src/org/eclipse/lsp4e/LanguageServerWrapper.java

@@ -59,6 +59,7 @@
 import org.eclipse.core.runtime.Adapters;
 import org.eclipse.core.runtime.Assert;
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.ILog;
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProduct;
 import org.eclipse.core.runtime.IProgressMonitor;
@@ -1017,8 +1018,11 @@ void registerCapability(RegistrationParams params) {
 	private void addRegistration(Registration reg, Runnable unregistrationHandler) {
 		String regId = reg.getId();
 		synchronized (dynamicRegistrations) {
-			Assert.isLegal(!dynamicRegistrations.containsKey(regId), "Registration id is not unique"); //$NON-NLS-1$
-			dynamicRegistrations.put(regId, unregistrationHandler);
+			if (dynamicRegistrations.containsKey(regId)) {
+				ILog.get().warn("A registration with id " + regId + " already exists. Unregistering may not fully work in this case.\n"); //$NON-NLS-1$ //$NON-NLS-2$
+			} else {
+				dynamicRegistrations.put(regId, unregistrationHandler);
+			}
 		}
 	}