Im using entityforms to make events on a Calendar. How can i prevent duplicated dates?
You could do this with Entity Rules, VBO, and Entityform if you are using 2.x(if not should upgrade).  

It actually would an advanced Rule but here are the general steps. 


Make a VBO View that has date time contextual filter. You would want to return any Entityform Submission that matched.
Make an Entity Rules Validation Rules.
Within Rule check the Number of results of the View from step #1 when you send it the date value of the current Entityform Submission.
If any results are returned then you have found a Entityform Submission that is for the existing date.


You would have to learn how to use Rules, Entity Rules, and VBO.