How do I display a field twice in a node's display?
Should be pretty easy if you use Display suite:

Add a dynamic field (admin/structure/ds/fields/manage_ctools) and select the entity type (node).



Your new field should be in the manage display tab of the entity type selected, activate it (put it in the section you want).

Click on the gear icon and then 'select content'



Then drill down in the options (node -> the field you would like to duplicate)



If you don't use display suite just print that field in a tpl.php file.