I created PR93461 for this issue: The following code causes a bogus "symbol is 
already defined" error (using git commit 
73380abd6b2783215c7950a2ade5e3f4b271e2bc):

module aModuleWithAnAllowedName

  interface
     module subroutine aShortName()
     end subroutine aShortName
  end interface
     
end module aModuleWithAnAllowedName

submodule (aModuleWithAnAllowedName) 
aSubmoduleWithAVeryVeryVeryLongButEntirelyLegalName

contains

  subroutine aShortName()
    call aSubroutineWithAVeryLongNameThatWillCauseAProblem()
    call aSubroutineWithAVeryLongNameThatWillCauseAProblemAlso()
  end subroutine aShortName
  
  subroutine aSubroutineWithAVeryLongNameThatWillCauseAProblem()
  end subroutine aSubroutineWithAVeryLongNameThatWillCauseAProblem

  subroutine aSubroutineWithAVeryLongNameThatWillCauseAProblemAlso()
  end subroutine aSubroutineWithAVeryLongNameThatWillCauseAProblemAlso
  
end submodule aSubmoduleWithAVeryVeryVeryLongButEntirelyLegalName



$ gfortran -v
Using built-in specs.
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/data001/abenson/Galacticus/Tools_Devel_Install/bin/../
libexec/gcc/x86_64-pc-linux-gnu/10.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-git/configure --prefix=/home/abenson/Galacticus/
Tools_Devel --enable-languages=c,c++,fortran --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.0.1 20200124 (experimental) (GCC) 


$ gfortran  -c symlength.F90 -o symlength.o -ffree-line-length-none -
frecursive  -pthread -Wall -fbacktrace -ffpe-trap=invalid,zero,overflow -fdump-
core -O3 -ffinite-math-only -fno-math-errno -fopenmp -g
/tmp/cc8B4Hmp.s: Assembler messages:
/tmp/cc8B4Hmp.s:20: Error: symbol 
`__amodulewithanallowedname.asubmodulewithaveryveryverylongbutentirelylegalname_MOD_asubroutinewithaverylongnamethatwillcauseaprobl' 
is already defined



The problem occurs because GFC_MAX_MANGLED_SYMBOL_LEN is set to 
GFC_MAX_SYMBOL_LEN*2+4, which is sufficient for a module name plus function name 
(plus the additional "_"'s that get prepended), but insufficient if a submodule 
name is included. The name then gets truncated and can lead to two different 
functions having the same (truncated) symbol name.

The fix is to increase this length to GFC_MAX_SYMBOL_LEN*3+5 - which allows for 
the submodule name plus the "." added between module and submodule names.

I've attached a patch for this which includes a new test case for this PR. The 
patch regression tests cleanly.

OK to commit?

-Andrew

-- 

* Andrew Benson: http://users.obs.carnegiescience.edu/abenson/contact.html

* Galacticus: https://github.com/galacticusorg/galacticus