The below trigger is used to warn the users if the Filename of an attachment already exists.

Trigger:

/*    Trigger to Warn for Duplicate Attachment Names    */


trigger DuplicateAttachment on Attachment (before insert) {

Set < String > setAttNames = new Set < String >();
Map < String, Attachment > mapAttachment = new Map < String, Attachment >();


for ( Attachment attachmnt:trigger.New ) {
setAttNames.add(attachmnt.Name);
}


for ( Attachment att : [ SELECT Id, Name FROM Attachment WHERE Name IN: setAttNames ] ) {
mapAttachment.put(att.Name, att);
}


for ( Attachment attachmnt : trigger.New ) {
if ( mapAttachment.containsKey(attachmnt.Name) ) { 
attachmnt.Name.addError(‘Duplicate Name. Filename already exists for attachment id ‘ + mapAttachment.get(attachmnt.Name) + ‘.’); 
}
}


}