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.
Don't pull your jeggings/skinny jeans on by the belt loop! - The Frugal Girl
3 Ways to Sew a Belt Loop - wikiHow
What is the back cinch? Denim FAQ answered by Denimhunters
Sewing Jeans Belt Loops the Easy Way - The Last Stitch
Belt Loop Repairs, the How & Why's Answered
8 Clever Small Space Tiny Closet Organizational Hacks • Save. Spend. Splurge.
How & Whys of Adding Belt Loops to Jeans
Complete Guide to Denim Terminology - Anatomy of Jeans
How to Wear Jean Chains: 9 Steps (with Pictures) - wikiHow
Unlike our other belts, this belt is designed to prevent tool belts or pant slippage. Simply run this hook closure pants belt through the belt loops
Hook & Loop Closure Pants Belt
Structural Denim Repair 101 - Pockets, Inseams, and Belt Loops
Is It Okay to Wear a Suit Without a Belt? – StudioSuits
Raised Belt Loops - Heddels
