Consider discarding repeated warnings

Writing tests for JDK-8287379 has highlighted the following problem: a tag misuse warning may be repeated multiple times:

Generating test1/out/B.html...
test1/src/B.java:1: warning: Tag @inheritDoc cannot be used in class documentation. It can only be used in the following types of documentation: method.
/** {@inheritDoc} */
    ^
test1/src/B.java:1: warning: Tag @inheritDoc cannot be used in class documentation. It can only be used in the following types of documentation: method.
/** {@inheritDoc} */
    ^
Generating test1/out/package-summary.html...
test1/src/B.java:1: warning: Tag @inheritDoc cannot be used in class documentation. It can only be used in the following types of documentation: method.
/** {@inheritDoc} */
    ^
Generating test1/out/package-tree.html...
Generating test1/out/overview-tree.html...
Building index for all classes...
Generating test1/out/allclasses-index.html...
test1/src/B.java:1: warning: Tag @inheritDoc cannot be used in class documentation. It can only be used in the following types of documentation: method.
/** {@inheritDoc} */
    ^
Generating test1/out/allpackages-index.html...
Generating test1/out/index-all.html...
test1/src/B.java:1: warning: Tag @inheritDoc cannot be used in class documentation. It can only be used in the following types of documentation: method.
/** {@inheritDoc} */
    ^
Generating test1/out/search.html...
Generating test1/out/index.html...
Generating test1/out/help-doc.html...
5 warnings

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This might require treatment similar to that for errors; see

com.sun.tools.javac.util.Log#shouldReport(javax.tools.JavaFileObject, int)
com.sun.tools.javac.util.Log#recorded