Repair leak on rain gutter
Ideally you'd disassemble the corner and replace parts so you can start fresh. Sealant over sealant is a low-percentage approach. 

Otherwise, it's a matter of scraping away as much of the old sealant as you can, cleaning the surface well (maybe with alcohol), and using one of the purpose-made sealants. It must be extremely flexible to handle movement due to temperature variation.