(debian-policy.info)Conflicting binary packages - Conflicts

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7.4 Conflicting binary packages - ‘Conflicts’
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When one binary package declares a conflict with another using a
‘Conflicts’ field, ‘dpkg’ will refuse to allow them to be unpacked on
the system at the same time.  This is a stronger restriction than
‘Breaks’, which prevents the broken package from being configured while
the breaking package is in the “Unpacked” state but allows both packages
to be unpacked at the same time.

If one package is to be unpacked, the other must be removed first.  If
the package being unpacked is marked as replacing (see Note: Overwriting
files and replacing packages - Replaces, but note that ‘Breaks’
should normally be used in this case) the one on the system, or the one
on the system is marked as deselected, or both packages are marked
‘Essential’, then ‘dpkg’ will automatically remove the package which is
causing the conflict.  Otherwise, it will halt the installation of the
new package with an error.  This mechanism is specifically designed to
produce an error when the installed package is ‘Essential’, but the new
package is not.

A package will not cause a conflict merely because its configuration
files are still installed; it must be at least “Half-Installed”.

A special exception is made for packages which declare a conflict with
their own package name, or with a virtual package which they provide
(see below): this does not prevent their installation, and allows a
package to conflict with others providing a replacement for it.  You use
this feature when you want the package in question to be the only
package providing some feature.

Normally, ‘Breaks’ should be used instead of ‘Conflicts’ since
‘Conflicts’ imposes a stronger restriction on the ordering of package
installation or upgrade and can make it more difficult for the package
manager to find a correct solution to an upgrade or installation
problem.  ‘Breaks’ should be used

   - when moving a file from one package to another (see Note:
     Overwriting files and replacing packages - Replaces.),

   - when splitting a package (a special case of the previous one), or

   - when the breaking package exposes a bug in or interacts badly with
     particular versions of the broken package.

‘Conflicts’ should be used

   - when two packages provide the same file and will continue to do so,

   - in conjunction with ‘Provides’ when only one package providing a
     given virtual facility may be unpacked at a time (see Note: Virtual
     packages - Provides.),

   - in other cases where one must prevent simultaneous installation of
     two packages for reasons that are ongoing (not fixed in a later
     version of one of the packages) or that must prevent both packages
     from being unpacked at the same time, not just configured.

Be aware that adding ‘Conflicts’ is normally not the best solution when
two packages provide the same files.  Depending on the reason for that
conflict, using alternatives or renaming the files is often a better
approach.  See, for example, Note: Binaries.

Neither ‘Breaks’ nor ‘Conflicts’ should be used unless two packages
cannot be installed at the same time or installing them both causes one
of them to be broken or unusable.  Having similar functionality or
performing the same tasks as another package is not sufficient reason to
declare ‘Breaks’ or ‘Conflicts’ with that package.

A ‘Conflicts’ entry may have an “earlier than” version clause if the
reason for the conflict is corrected in a later version of one of the
packages.  However, normally the presence of an “earlier than” version
clause is a sign that ‘Breaks’ should have been used instead.  An
“earlier than” version clause in ‘Conflicts’ prevents ‘dpkg’ from
upgrading or installing the package which declares such a conflict until
the upgrade or removal of the conflicted-with package has been
completed, which is a strong restriction.