Google indexing in uppercase & lowercase?

Why are they doing this?

[jake66]

Within my site, I link to my site only Like This, not like this also (upper-case & lower)...yet google has begun indexing both!

As well, they are now indexing URLs such as search queries like: search.php?my+search+term as:
search.php?my%20search%20term

Since I noticed this problem, I put some additional checks in my scripts to stop this.
Which is better?
404 the lower-case & URLs with %20 in them.. or 301 to their proper location?

My site has been online since 2005 and this is the first I've come across this. How can I figure out why google begun indexing my site like this, all of a sudden? (they just appeared in webmaster tools the other day!)

[jdMorgan]

It could be a competitor has discovered that you had done nothing to prevent this, and is exploiting this vulnerability in an attempt to give your pages a duplicate-content 'penalty'...

Or it could be caused by just a few bad links posted here and there on the Web.

But anything that you do not explicitly prevent might very well happen; Do not rely on the search engines to "figure it out somehow." Canonicalize everything!

I'd 301 them, myself.

Jim

[edited by: jdMorgan at 9:34 pm (utc) on Dec. 3, 2008]