There are 3 possibilities I can think of. One is that, although your crochet pattern is your own, you may also have it up elsewhere on the web (yes, even if it's under your name). If so, you will either need to remove it from Hubpages or from the other site, as Hubpages does not allow 2 copies of the same work anywhere.
The other possibility is that Hubpages *thinks* your pattern is a duplicate of another, even though it isn't. Remember it's an automated computer program that scans hubs for duplicates on the web - and in the case of a knitting or crochet pattern, instructions can look like random jargon (e.g. ch3 incr1 , etc) - so it might be ignoring the jargon and finding that the rest of it is remarkably similar to something already there - or maybe a few lines of your pattern jargon will be similar to existing patterns. This will especially get picked up on if you don't have much other introductory text. If this is the case, try adding an introductory paragraph or two to see if that helps (however, be aware it may not always help).
The third possibility is that someone else has plagiarized your pattern elsewhere on the web. Try doing a Google search for some of the exact phrases in your pattern - either in the introductory wording (if you have an intro), or from the pattern itself. Try a few different phrases of several words each. Remember to surround the phrase by quotation marks when googling it, to ensure you are searching for the exact phrase. If it turns out someone has plagiarized it, don't get over-stressed - there is a way to deal with it: you need to bring it to the attention of Hubpages (not sure who, a moderator I guess?). Then you will have to get the plagiarized copy taken down. Try to enlist help of HP moderator in doing that if possible. It is usually pretty easy to prove whose was there first by using the Wayback machine. http://www.archive.org/web/web.php Above all, it's best NOT to unpublish a hub of yours that someone else has plagiarized (because your hub may then not show up on the Wayback machine at the critical time in question).
If none of these apply, contact HP and explain what is going on.
It is always upsetting when you receive a message that seems incorrect, but just follow through on the possibilities I listed and hopefully you will get it resolved soon. Don't give up, and best wishes!