Conflicting packages : same priority (nix-env)

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I tried to build the package keepasshttp in my non-NixOS nix-env.
But this failed with the following error message

nix-env -i keepasshttp

warning: there are multiple derivations named 'keepasshttp-1.8.4.2'; using the first one
installing 'keepasshttp-1.8.4.2'
these 2 derivations will be built:
  /nix/store/dd71xkx6vcvv3riikjyvhfw0fkjnzkbz-builder.pl.drv
  /nix/store/pzzc3jnfccbxx6qyr12655kkjx720x2k-keepasshttp-1.8.4.2.drv
these 12 paths will be fetched (2.12 MiB download, 9.04 MiB unpacked):
  /nix/store/18ggindri5xm8zbdpzy7x2927z1lg5gc-stdenv-linux
[..]
copying path '/nix/store/4b7fgrll7f7aq3x78fgc8cwjphls6hg7-keepasshttp-1.8.4.2' from 'https://cache.nixos.org'...
[..]
copying path '/nix/store/18ggindri5xm8zbdpzy7x2927z1lg5gc-stdenv-linux' from 'https://cache.nixos.org'...
building '/nix/store/dd71xkx6vcvv3riikjyvhfw0fkjnzkbz-builder.pl.drv'...
building '/nix/store/pzzc3jnfccbxx6qyr12655kkjx720x2k-keepasshttp-1.8.4.2.drv'...
created 60 symlinks in user environment
building '/nix/store/kkpazsgmn6q00s58zl1spahr09sv5ck5-user-environment.drv'...
error: error: --- Error ------------------------------------------------------------------------------------------------------------------- nix-env
packages '/nix/store/sp19m55hkzd1v705m8sv5mjn47hdcjba-keeagent-0.12.0/share/man/man5/mdoc.5.gz' and '/nix/store/gfkwrx5qz1yx8bziqig0634wxnvpap0y-mono-5.20.1.27/share/man/man5/mdoc.5.gz' have the same priority 5; use 'nix-env --set-flag priority NUMBER INSTALLED_PKGNAME' to change the priority of one of the conflicting packages (0 being the highest priority)
error: --- Error ------------------------------------------------------------------------------------------------------------------- nix-env
builder for '/nix/store/kkpazsgmn6q00s58zl1spahr09sv5ck5-user-environment.drv' failed with exit code 1

This error appears only if I already installed another keepass-plugin (i.e. otpkeyprov, keepassrpc). Only keeagent seem’s not to be affected.

I see that there are two derivations of this package. I guess it’s because the package maintainers decided to use stdenv in conjunction with buildenv ? Is that the cause? And if so, how can I solve that issue?

The packages are looking like this:

{ stdenv, buildEnv, fetchFromGitHub, mono }:

let
  drv = stdenv.mkDerivation {
    src = fetchFromGitHub {
      [..]
    };
    meta = {
      [..]
    };
    pluginFilename = [..];
    installPhase = ''
      [..]
    '';
  };
in
  # Mono is required to compile plugin at runtime, after loading.
  buildEnv { name = drv.name; paths = [ mono drv ]; }

But what I don’t get is, that this package builds fine (w/o errors) on NixOS 20.09.
On both platforms I am using nix-env unstable (3.0pre20200829_f156513)
Channel on NixOS is: http://nixorg.irg/channels/nixos-20.09
Channel on Nix is: nixpkgs-unstable release nixpkgs-21.11pre326916.7053541084b

P.S.: I don’t think setting a package priority would eliminate the cause of this issue - but it could be some kind of ugly workaround.