Symptom: when installing or upgrading, you get an error message such as $ 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. The cause is that two installed packages in the user environment have overlapping filenames (e.g., xml/dtd/docbook/calstblx.dtd. 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 docbook-xml, so nix-env -i will try to install both. The default user environment builder has no way to way to resolve such conflicts, so it just gives up. Solution: remove one of the offending packages from the user environment (if already installed) using nix-env -e, or specify exactly which version should be installed (e.g., nix-env -i docbook-xml-4.2).