Why are there duplicate game entries in my Steam library?

Seeing as that game is a SteamPlay game, the duplicates are Mac versions of the game.

The easiest way to hide any game is to categorise your games and only show those you want to see, or just hide those that are not installed. You can do so by clicking All Games and clicking the category you want to see.

Alternatively, at least in library mode, if you give them a certain category they will be put in a different section which you can hide by clicking.

I submitted a question to Steam Support a while back on this ... got the following response ...

Hello Bryan,

Thank you for contacting Steam Support.

Currently, all copies of Civilization IV games have double listings (One for PC and one for Mac).

We are aware of this issue and working to correct it in the near future.

Our apologies for the inconvenience.

If you have any further questions, please let us know.

... looks like Arda Xi is correct!