Lucia ( Crochet Jane ;) ) posted a code fix for the trailing slashes problem (If you ain't subscribed to Lucia's blog, there is just something wrong with you).

The Duplicate Page Problem

Basically the problem is this. If your blog will accept http://yourblogdomain.tld/some-post/
and also accept
http://yourblogdomain.tld/some-post
(the first has a trailing slash and the second doesn't), then you are dividing your link juice between "two" identical pages, and Google has to figure out which one they want to use. They might decide on the one you wish they hadn't.

Lets say you are getting oodles of links to the without slashes one, and Google notices that all of your internal links go to the with slashes one. Well, they could decide to prefer the one you are linking to and thus rank your page according to it's link juice and not the one everyone else is linking to.

That's a bad thing. Trust me. Or ask Lucia, she'll tell you the same thing and she has a more trust able face than me.

The Redirection Solution

So, she posted some code you can put into your blog theme's header.php template that will issue a 301 redirect to the trailing slash version of the page if anyone tries to load the no trailing slash version. A 301 redirect is a "Permanently Moved" redirect which tells Google (and anyone else) that they should stop using the page they tried to use and use this other one instead.

So now when Google finds a link to
http://yourblogdomain.tld/some-post (no slash)
they will get the 301 redirection to
http://yourblogdomain.tld/some-post/ (with slash)
and they will automatically start preferring that version. They will also take and Link Love to the no slash page and apply it to the slash page instead, which means that it gets ranked no matter how anyone links to it.

You have options now. You can visit Lucia and get her code (which is my recommendation), or if you are not comfortable with modifying your template files (it is easy though), you can try using the Permalink Redirect Plugin. I've not tried it myself, but I saw a recommendation for it from John Godley, and that's good enough for me.

Another Redirection Fix

Incidentally, there is an identical problem at the other end of your URIs. Namely it's the whole www. or not thing. If some links go to
http://www.yourblogdomain.com/
and others go to
http://yourblogdomain.com/
it can cause the same problem. John Godley has a redirection plugin that lets you manage all kinds of cool redirections from your dashboard. One of the configuration options is to either force or strip the www from requests through a 301 redirection.

Pretty cool, huh? And that's only a tiny bit of what it does.