Custom rule is invoked twice for a single script

I am writing custom rules and I see an error reported twice when there should be just one report.

I invoke script analyzer like this:

#11 PS> Invoke-ScriptAnalyzer -CustomRulePath D:\gh\PesterAnalyzerRules -Path a.ps1

RuleName                            Severity     ScriptName Line  Message
--------                            --------     ---------- ----  -------
PesterAnalyzerRules\Measure-ShouldT Information  a.ps1      3     Pipe a script block to 'Should Throw' or 'Should Not Throw'
hrow
PesterAnalyzerRules\Measure-ShouldT Information  a.ps1      3     Pipe a script block to 'Should Throw' or 'Should Not Throw'
hrow


@ D:\xtmp
#12 PS> cat a.ps1

Describe "a" {
    "b" | Should Throw
}

In a debugger, I noticed my rule is getting called on two different threads.

Start by reproducing the issue with Invoke-ScriptAnalyzer, -CustomRulePath, and the a.ps1 example shown in the report. Trace the custom-rule invocation across the two observed threads and verify that the rule produces one report for the single script occurrence; the command output should contain only one diagnostic.