How to stop Mac to convert typing double dash to emdash?

(on Sierra 10.12, this is now in the Keyboard control panel)

Go to the system preferences and choose "Language & Text"

In the "Text" tab you will find the list of substitutions.

If you want to stop -- from being turned into — (emdash) you need to use the + button and add a new rule to replace -- with -- (replace with itself)

Or, right click in a text field and go to "Substitutions" and disable "Smart Dashes"

I encountered this same problem on Mavericks (10.9), where the fix has changed slightly. Go to System Preferences, then "Language & Region" then click the "Keyboard Preferences..." button and to go to "Text" tab. It is no longer a substitution, however, but instead on the right-hand side of the window there is a tickbox "Use smart quotes and dashes". Unticking this stops it changing two hyphens into an en-dash.

One little gotcha: on 10.9.5 with TextEdit (and probably others) you must exit then restart your editor to have this change take effect.

In 11.11 this is under System Preferences > Keyboard. There is a checkbox for Use smart quotes and dashes.

Keyboard pref in 11.11