It's not a design flaw, or a backup in case the main loop breaks. It actually serves a purpose. Here are some theories and the real reason for the loops.
Installing Jean Belt loops, Hemming Jeans & Sewing Buttonhole
Denim shorts under magnification. Trouser zipper, belt loops and other small items in jeans. Light background Stock Photo - Alamy
How to Sew Belt Loops - Easily & Quickly
Is It Okay to Wear a Suit Without a Belt? – StudioSuits
Choosing a Belt to Wear with Jeans
Installing Jean Belt loops, Hemming Jeans & Sewing Buttonhole
Jean rivets: What are those little studs actually for?
3 Ways to Sew a Belt Loop - wikiHow
How to Make Jeans Smaller: TikTok Hack
Belt Loops Vs. Side Adjusters: Which do You Choose?
Toddler Snap Belts
BELT LOOP - 小物
How to Fix a Ripped Belt Loop on a Pair of Jeans - iFixit Repair Guide
Denim Jeans Fading Guide Explains Terminology