If you have already restored the site collection (e.g. http://sp2010vm/) on the given web application and if you try to restore same site collection at the different path (e.g. http://sp2010vm/sites/projectinit) in the same web application and same content database, Restore-SPSite with throw error – “The operation that you are attempting to perform cannot be completed successfully. No content databases in the web application were available to store your site collection. The existing content databases may have reached the maximum number of site collections, or be set to read-only, or be offline, or may already contain a copy of this site collection. Create another content database for the Web application and then try the operation again”.
Another scenario is if you have already restored the site collection (e.g. http://sp2010vm/sites/PI) on the given web application and for some reason, deleted the site collection, and try to restore same site collection (e.g. http://sp2010vm/sites/projectinit) in the same web application and same content database, Restore-SPSite with throw same error.
The reason for this is – If you have already restored the site collection on given web application once, you can’t restore the same site collection in same content database in the given web application as the different site collection location. Microsoft uses the source site collection GUIDs during the installation. If you delete the site collection from the web application, it leaves the GUIDs behind, and if you try to restore same site collection (same GUID) into same web application and content database, it would throw an error. To resolve the issue, new content database needs to be created in the web application to restore the site collection.
An article referencing the MOSS 2007, still applies to SharePoint 2010
Although above process adds one more content database in the backup/restore strategies, it would be cleaner solution. There is a work around to restore the site collection in the same content database which I haven’t validated for the SharePoint 2010.