nix-super/doc/manual/troubleshooting/collisions-nixenv.xml

<section xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"
      version="5.0"
      xml:id="sec-collisions-nixenv">

<title>Collisions in <command>nix-env</command></title>

<para>Symptom: when installing or upgrading, you get an error message such as

<screen>
$ nix-env -i docbook-xml
...
adding /nix/store/s5hyxgm62gk2...-docbook-xml-4.2
collision between `/nix/store/s5hyxgm62gk2...-docbook-xml-4.2/xml/dtd/docbook/calstblx.dtd'
  and `/nix/store/06h377hr4b33...-docbook-xml-4.3/xml/dtd/docbook/calstblx.dtd'
  at /nix/store/...-builder.pl line 62.</screen>

</para>

<para>The cause is that two installed packages in the user environment
have overlapping filenames (e.g.,
<filename>xml/dtd/docbook/calstblx.dtd</filename>.  This usually
happens when you accidentally try to install two versions of the same
package.  For instance, in the example above, the Nix Packages
collection contains two versions of <literal>docbook-xml</literal>, so
<command>nix-env -i</command> will try to install both.  The default
user environment builder has no way to way to resolve such conflicts,
so it just gives up.</para>

<para>Solution: remove one of the offending packages from the user
environment (if already installed) using <command>nix-env
-e</command>, or specify exactly which version should be installed
(e.g., <literal>nix-env -i docbook-xml-4.2</literal>).</para>

<!-- FIXME: describe priorities -->

</section>