Been there, done that, took the photo and got a t-shirt to prove it. You have to realize when you get into the microstock market that almost every possible photo has (probably) already been shot once. Do not let this discourage you though, you can not really avoid it anyway. Just always keep in mind that you should try to make your photo stand out above the rest, so it appeals to your potential buyer.
As a microstock photographer you should also take note of what the agencies your uploading to want to have in their portfolio. Nobody wants to see your shadow, pet or feet. Most microstock agencies even provide lists of things that they do and do not need. Below I have included the “want” and “want not” lists of Fotolia and iStockphoto.

Want: Health and medical field, Business related photos, People in activities, Controversial issues, Religious faith, Technology photos, Objects, Close up shots, Under water photos.
Want not: Dogs, Cats, Ducks, Insects, Flowers, Sunsets, Nature, Fine arts.
Source: Fotolia.com

Want: Corporate shots, Groups & Teams, Concept Stock, Sports, Rare Artifacts, Illustrations, Holiday & Seasonal Themes, Fashion, Analogue and Grunge, Food and Beverages.
Want not: Your dog or cat, Flowers, Flags, Photos of your feet, socks, etc., Your immediate environment, Light Blurs, Brick walls, Close-ups of fire, Close-ups of eyes, Symbols (!$%@), Sunsets & clouds, Forest Snap Shots, Airplane Wings (& out the window shots), Your Shadow, Overly simple 3-D & Bryce renderings & Photoshop backgrounds, Photoshop Backgrounds, Overly simple text.
Source: iStockphoto.com