For me It was duplicate in «ValleyRackFree» although I already have the same folder named «Valley». Most likely I compiled them earlier(before these modules appeared in library) from the source code and forgot to delete the folder ¯_(ツ)_/¯
[2.053 info src/plugin.cpp:156] Loaded plugin Valley v1.0.1 from /Users/shantarli/Documents/Rack/plugins-v1/Valley
[2.071 warn src/plugin.cpp:160] Could not load plugin /Users/shantarli/Documents/Rack/plugins-v1/ValleyRackFree: Plugin Valley is already loaded, not attempting to load it again
[2.094 fatal src/main.cpp:38] Fatal signal 6. Stack trace:
13: 1 Rack 0x0000000107cbae29 _ZL18fatalSignalHandleri + 41
12: 2 libsystem_platform.dylib 0x00007fff5d776b5d _sigtramp + 29
11: 3 ??? 0x0000000100000000 0x0 + 4294967296
10: 4 libsystem_c.dylib 0x00007fff5d6306a6 abort + 127
9: 5 libsystem_malloc.dylib 0x00007fff5d73f077 malloc_vreport + 545
8: 6 libsystem_malloc.dylib 0x00007fff5d73ee38 malloc_report + 151
7: 7 plugin.dylib 0x000000010e3726e3 _ZN4rack6plugin5ModelD2Ev + 83
6: 8 plugin.dylib 0x000000010e3ac9fe _ZZN4rack11createModelI9Topograph15TopographWidgetJEEEPNS_6plugin5ModelERKNSt3__112basic_stringIcNS6_11char_traitsIcEENS6_9allocatorIcEEEEEN6TModelD0Ev + 14
5: 9 Rack 0x0000000107d161fe _ZN4rack6plugin6PluginD2Ev + 46
4: 10 Rack 0x0000000107cc174b _ZN4rack6pluginL10loadPluginENSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEE + 2859
3: 11 Rack 0x0000000107cc044b _ZN4rack6plugin4initEv + 1131
2: 12 Rack 0x0000000107cba840 main + 912
1: 13 Rack 0x0000000107c52034 start + 52
0: 14 ??? 0x0000000000000001 0x0 + 1