I have a ton of mods installed, including the whole of bz, k2, 248k and se 6. When I include the mod RealisticReactors_3.1.4.zip, I get

11.774 Error ModManager.cpp:1558: Error while running setup for recipe prototype "kr-matter-plant" (recipe): Duplicate item ingredients are not allowed (fu_materials_KFK exists 2 or more times).

The mods listed are:
Krastorio2 (1.3.4)
bzaluminium (0.4.2)
space-exploration-postprocess (0.6.18)
248k (1.0.29)

If I remove the realistic reactors mod, it loads. This is entirely repeatable. I can only think that RealisticReactors alters the mod initialization order somehow that breaks an accidental data modification dependency somewhere. I have pasted in the log below. I am not really sure how to debug this.

11.774 Error ModManager.cpp:1558: Error while running setup for recipe prototype "kr-matter-plant" (recipe): Duplicate item ingredients are not allowed (fu_materials_KFK exists 2 or more times).

Sorry - I don't seem to be able to attach the file, and it is too long to fit into a post here. The logged mods are below - as I say, removing RealisticReactors makes it load:
["248k"] = "1.0.29",
Aircraft = "1.8.4",
["Aircraft-plus-modpack"] = "0.0.1",
ArmouredBiters = "1.1.6",
AsphaltPaving = "1.3.1",
AutoDeconstruct = "0.3.3",
Automatic_Train_Painter = "1.1.4",
EditorExtensions = "1.13.4",
EpicArtillerySounds = "1.1.1",
FluidWagonColorMask = "1.1.0",
HelicopterRevival = "0.2.1",
["Hover-Car"] = "0.2.15",
InsertionRequester = "0.1.0",
Krastorio2 = "1.3.4",
Krastorio2Assets = "1.2.0",
LogisticRequestManager = "1.1.23",
Mining_Drones = "1.1.10",
ModuleInserterER = "5.2.5",
Noxys_Trees = "0.4.2",
PickerDollies = "1.2.1",
PlutoniumEnergy = "1.4.3",
QueueToFrontNG = "0.18.3",
RateCalculator = "2.4.4",
Raven = "0.1.12",
RealisticFusionPower = "1.8.15",
RealisticFusionWeaponry = "1.5.8",
RealisticReactors = "3.1.4",
RecipeBook = "3.4.5",
RemoveTintedConcrete = "0.0.1",
SpidertronEnhancements = "1.8.7",
TLBE = "1.4.4",
TinyStart = "1.1.1",
TinyStartCraftablesupdate = "1.0.4",
["Todo-List"] = "19.2.0",
["Transport-Drones-On-Asphalt"] = "0.1.1",
Transport_Drones = "1.0.9",
["True-Nukes"] = "0.2.13",
["True-Nukes-Graphics"] = "0.0.1",
["aai-containers"] = "0.2.11",
["aai-industry"] = "0.5.19",
["aai-signal-transmission"] = "0.4.7",
["alien-biomes"] = "0.6.7",
["alien-biomes-hr-terrain"] = "0.6.1",
["artillery-bombardment-remote-reloaded"] = "1.2.0",
base = "1.1.65",
betterCargoPlanes = "1.3.7",
["big-data-string"] = "1.0.1",
bobinserters = "1.1.5",
["bullet-trails"] = "0.6.2",
["bz-dsr-bridge"] = "0.5.0",
bzaluminum = "0.4.2",
bzcarbon = "0.4.0",
bzfoundry = "0.2.5",
bzgas = "0.0.16",
bzlead = "1.2.11",
bzsilicon = "0.11.2",
bztin = "0.1.2",
bztitanium = "1.2.5",
bztungsten = "0.6.6",
bzvery = "0.4.0",
bzzirconium = "0.6.8",
["combat-mechanics-overhaul"] = "0.6.22",
["equipment-gantry"] = "0.1.5",
["even-distribution"] = "1.0.10",
factoryplanner = "1.1.51",
flib = "0.10.1",
["grappling-gun"] = "0.3.2",
inbuilt_lighting = "20.1.11",
informatron = "0.2.2",
jetpack = "0.3.5",
["nco-SpaceShuttle"] = "0.0.5",
["nixie-tubes"] = "1.1.3",
["production-monitor"] = "1.1.1",
qol_research = "3.3.0",
railloader = "1.1.6",
["recursive-blueprints"] = "1.2.6",
robot_attrition = "0.5.12",
["rusty-locale"] = "1.0.16",
["shield-projector"] = "0.1.6",
simhelper = "1.1.4",
["space-exploration"] = "0.6.82",
["space-exploration-graphics"] = "0.6.11",
["space-exploration-graphics-2"] = "0.6.1",
["space-exploration-graphics-3"] = "0.6.1",
["space-exploration-graphics-4"] = "0.6.2",
["space-exploration-graphics-5"] = "0.6.1",
["space-exploration-menu-simulations"] = "0.6.8",
["space-exploration-postprocess"] = "0.6.18",
stdlib = "1.4.7",
textplates = "0.6.9"

So, it seems like the bug exists between:

Krastorio2
Space Exploration
248k
RealisticReactors
PlutoniumEnergy

If I enabled those 5 mods (and all their dependencies) I get the same error. Probably needs to be addressed in 248k, but not positive about that.

OK - thanks - it must be an odd edge case for recipe updating I guess. I'll consider it closed for bzaluminium. Did it only happen for you with these 5 and deps with bzaluminium, or was it reproducible with out it?

Yes just those 5, reproducible without bzaluminum.