// Including a fragment more than once in the same document

A document can include the same file any number of times.
The problem comes if there are IDs in the included file; the output document (HTML or DocBook) will then have duplicate IDs which will make it not well-formed.
To fix this, you can reference a dynamic variable from the main document in the ID.

For example, let's say you want to include the same subsection describing a bike chain in both the operation and maintenance chapters:

----
= Bike Manual

:chapter: operation
== Operation

\include::fragment-chain.adoc[]

:chapter: maintenance
== Maintenance

\include::fragment-chain.adoc[]
----

Write [.path]_fragment-chain.adoc_ as:

----
[id='chain-{chapter}']
=== Chain

See xref:chain-{chapter}[].
----

The first time the [.path]_fragment-chain.adoc_ file is included, the ID of the included section resolves to `chain-operation`.
The second time the file included, the ID resolves to `chain-maintenance`.

In order for this to work, you must use the long-hand forms of both the ID assignment and the cross-reference.
The single quotes around the variable name in the assignment are required to force variable substitution (aka interpolation).