Attempt to create an instance of field that already has an instance of that field how can i check if an instance of a field has already been created so that the my_module condition doesn't try to create it again?
something like 

if(!instance_exist){
//do abc
}
The function you are looking for is field_info_instance($entity_type, $field_name, $bundle);

for example:

field_info_instance('node', 'field_tags', 'article');


will return the field instance if a field 'field_tags' exists on nodes of the content type 'article', NULL otherwise.