When you load a world with a data pack using multiple namespaces (ie, two folders in the data folder), the game takes forever to load the world and crash after a while.

It says "duplicate recipe ignored" in the crash report event if I use only one recipe in the pack (the other namespace contains only advancements and functions).

Report:
---- Minecraft Crash Report ----
// Would you like a cupcake? yes please

Time: 30/11/17 20:26
Description: Exception in server tick loop

java.lang.IllegalStateException: Duplicate recipe ignored with ID folder_2:elytra
at aoe.a(SourceFile:70)
at aoe.a(SourceFile:52)
at sd.c(SourceFile:132)
at sd.a(SourceFile:115)
at net.minecraft.server.MinecraftServer.a(SourceFile:1301)
at net.minecraft.server.MinecraftServer.a(SourceFile:330)
at ckz.a(SourceFile:81)
at ckz.d(SourceFile:128)
at net.minecraft.server.MinecraftServer.run(SourceFile:463)
at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:
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– System Details –
Details:
Minecraft Version: 17w48a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 307801160 bytes (293 MB) / 407150592 bytes (388 MB) up to 2134114304 bytes (2035 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx2G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 98
Profiler Position: N/A (disabled)
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.