Android OS Storage Usage
With root take a comparison of space usage on both devices by executing du -h /data | sort -h. Without root you can rely on figures reported by filesystem by running df -h.

From the updated screenshot you have attached, no partition is duplicate mounted. If you are talking about tmpfs, it's a volatile filesystem which lives in RAM and is vanished on reboot. It isn't part of your persistent storage.

Your /data and /system added make 29GiB. That's what you get on a 32GB device. Up to 2GiB difference comes because of Gigabyte (GB) and Gibibyte (GiB). Also there are usually 50+ partitions on an Android device which aren't mounted (except a few filesystems). And there's free space left by manufacturers for over-provisioning of flash memory.

For details see: How disk space is used on Android device?