Edit: a full-fledged issue report.

Minecraft Version

1.12.2

Mod Loader

Forge

Mod Loader Version

Forge Loader 14.23.5.2860

Mod Version

WearableBackpacks-1.12.2-3.1.4

Describe the Issue

I'm playing on RLCraft, sometimes when I die or someone who is playing dies with a backpack on, the backpack duplicates. I am pretty sure this is an issue with either Corpse Complex, Hardcore Revival (added it on myself), or both. I made it so that you always keep your backpack on death, it must be something with Hardcore Revival that's messing this up. When downed and then not rescued in time, the backpack they wear is dropped. All items are still inside. When they respawn, that same backpack is still on their back, same items. CorpseComplex makes it so the backpack stays on death, I turned that on in its config.

Logs

Nothing in the logs show any kind of indication of a bug happening. This is a list of the mods I am playing with. Essentially, its just RLCraft. I added a few mods.

minecraft, mcp, FML, forge, creativecoredummy, entity_culling, itemphysic, ivtoolkit, foamfixcore, tnt_utilities_core, adhooks, advanced-fishing, aireducer, alcatrazcore, antiqueatlas, antiqueatlasoverlay, anvilpatch, aquaacrobatics, aquaculture, armorunder, astikorcarts, autoreglib, base, battletowers, baubles, bedbreakbegone, mujmajnkraftsbettersurvival, bettercaves, bettercombatmod, betterfoliage, bettermineshafts, betternether, betterquesting, bettertabs, blockoverlayfix, bloodmoon, bnbgaminglib, bountifulbaubles, bountiful, bq_msi, bqtweaker, callablehorses, carrots, carryon, charm, chunkanimator, classyhats, collisiondamage, comforts, compatskills, contenttweaker, controlling, coralreef, corpsecomplex, cosmeticarmorreworked, craftablechainmail, craftablehorsearmour, craftpresence, ctgui, crafttweaker, crafttweakerjei, creativecore, custommainmenu, cyclicmagic, defiledlands, disenchanter, dldungeonsjbg, dsurround, dynamictrees, dynamictreesdefiledlands, dttraverse, elenaidodge, enchdesc, enhancedvisuals, familiarfauna, fbp, fantasticlib, firstaid, fishingmadebetter, foamfix, foodexpansion, forgeendertech, forgelin, forgottenitems, friendlyendermite, cfm, fxcontrol, globalgamerules, googlyeyes, grapplemod, hardcorerevival, iceandfire, ichunutil, infernalmobs, inspirations, inventorytweaks, iseedragons, itlt, jei, levelup2, librarianliblate, librarianlib, loadingscreens, locks, loottweaker, lostcities, lycanitesmobs, mainmenuscale, mantle, mcwbridges, testdummy, mobends, modtweaker, mousetweaks, mtlib, multimine, neat, notreepunching, particleculling, pigstep, placebo, playerbosses, potioncore, potionfingers, qualitytools, quark, realistictorches, reborncore, reccomplex, reskillable, resourceloader, rltweaker, roguelike, roughtweaks, ruins, rustic, scalinghealth, sereneseasons, setbonus, shieldbreak, silentlib, simpledifficulty, sit, snowrealmagic, somanyenchantments, soundfilters, spartandefiled, spartanfire, spartanhudbaubles, spartanshields, spartanweaponry, spawnercontrol, srparasites, bq_standard, switchbow, tickprofiler, tnt_utilities, toastcontrol, toolbelt, traverse, xat, trumpetskeleton, tschipplib, tumbleweed, variedcommodities, waystones, wearablebackpacks, wolfarmor, xpfromharvest, xpbook, yungslaw, dshuds, llibrary, orelib, phosphor-lighting

Do you use any performance-enhancing mods (e.g. OptiFine)?

I am using OptiFine.