Why do some proteins always show as double bands on Western blot?

There can be different reasons for double bands detected on your blots:

Target protein is degraded during extraction procedure. Make use fresh buffers and protease inhibitors are used. Work in the cold and promptly.

Protein sample is stored for too long and deteriorated during storage. Maximum time of storing protein samples is up to 6 months.

Target protein is present as several isoforms due to alternative splicing or gene duplicates, which can differ from each other by just a few kDa. Check this up on the sequence level.

Target protein which you aim to detect, is subjected to PTMs (Posttranslational Modifications) as phosphorylation, glycosylation, ubiquitination. Check in available database, if your target protein can be a subject of PTMs. These modifications can be blocked by treating samples with phosphatases or glycosidases.

Artifacts due to errors in the procedure: gel polymerization and running buffers. Double check gel quality and prepare fresh buffers.