How do I make a page available via only a single URL?
That's what the &lt;link rel="canonical"&gt; meta tag is for.

I'm not sure where you've got the information that this meta tag is not good for SEO, but I'm afraid your source is incorrect; the exact opposite is true. This is what Google have to say on the matter:


  Carpe diem on any duplicate content worries: we now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.


As you can see this is what Google recommend you do to avoid the problem of SEO penalties for duplicate content. This information is several years old and hasn't changed.

To my knowledge the following modules will add a canonical link to your documents:


Global Redirect
Redirect
Meta Tag


There may well be more though. The 2 redirect modules above will probably help you out even further, as they will automatically perform a 301 redirect to the canonical URL when any of the others are visited. Google also encourages the use of such headers in this situation.

Be sure to read the whole Official Google Webmaster blog post linked to above to understand why this method is the correct one, it's pretty important in terms of SEO these days. 

Oh, and if I were you I'd be suspicious of anything else your original source for this tells you...they obviously don't know much about SEO!

Further Reference:


About rel="canonical" on Google Webmaster Tools
Canonical URL Tag - The Most Important Advancement in SEO Practices Since Sitemaps on SEOMoz
Google, Yahoo &amp; Microsoft Unite On “Canonical Tag” To Reduce Duplicate Content Clutter on Search Engine Land


And there are literally dozens more...search Google for "SEO canonical".