Auto merge of #11604 - pkgw:resolver-unification-docs, r=ehuss

book: describe how the current resolver sometimes duplicates deps

### What does this PR try to resolve?

This updates the book to document the behavior described in #9029, where sometimes Cargo will duplicate a dep when it doesn't have to.

### How should we test and review this PR?

Doc-only change; someone with knowledge of the resolver should read and assess.

Author: bors

src/doc/src/reference/resolver.md

@@ -7,8 +7,10 @@ result of the resolution is stored in the `Cargo.lock` file which "locks" the
 dependencies to specific versions, and keeps them fixed over time.
 
 The resolver attempts to unify common dependencies while considering possibly
-conflicting requirements. The sections below provide some details on how these
-constraints are handled, and how to work with the resolver.
+conflicting requirements. It turns out, however, that in many cases there is no
+single "best" dependency resolution, and so the resolver must use heuristics to
+choose a preferred solution. The sections below provide some details on how
+requirements are handled, and how to work with the resolver.
 
 See the chapter [Specifying Dependencies] for more details about how
 dependency requirements are specified.
@@ -490,6 +492,35 @@ break the build.
 The following illustrates some problems you may experience, and some possible
 solutions.
 
+### Unexpected dependency duplication
+
+The resolver algorithm may converge on a solution that includes two copies of a
+dependency when one would suffice. For example:
+
+```toml
+# Package A
+[dependencies]
+rand = "0.7"
+
+# Package B
+[dependencies]
+rand = ">=0.6"  # note: open requirements such as this are discouraged
+```
+
+In this example, Cargo may build two copies of the `rand` crate, even though a
+single copy at version `0.7.3` would meet all requirements. This is because the
+resolver's algorithm favors building the latest available version of `rand` for
+Package B, which is `0.8.5` at the time of this writing, and that is
+incompatible with Package A's specification. The resolver's algorithm does not
+currently attempt to "deduplicate" in this situation.
+
+The use of open-ended version requirements like `>=0.6` is discouraged in Cargo.
+But, if you run into this situation, the [`cargo update`] command with the
+`--precise` flag can be used to manually remove such duplications.
+
+[`cargo update`]: ../commands/cargo-update.md
+
+
 ### SemVer-breaking patch release breaks the build
 
 Sometimes a project may inadvertently publish a point release with a