Release Channel (137.0.2) const variable useChrome.js seeing Multiple "SyntaxError: redeclaration of const"

[WoofGrrrr]

I just switched from ESR (128.*) to Release Channel (137.0.2)

The userChrome.js I've been using for well over a year is now getting multiple "SyntaxError: redeclaration of const" errors when it runs, but not always:

UPDATE: This seems to happen ONLY when the extension is installed. I have been re-installing while trying to work around the multiple CSP (Content Security Policy) errors I've been seeing, discussed in Issue 5

14:19:41.758 SyntaxError: redeclaration of const WINDOW_ID_COMMON_DIALOG
    <anonymous> file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/chrome/userChrome.js:1
    paint jar:file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/extensions/userChromeJS@kamens.us.xpi!/experiments.js:210
    checkAndRunExtensionCode resource:///modules/ExtensionSupport.sys.mjs:221
    _checkAndRunMatchingExtensions resource:///modules/ExtensionSupport.sys.mjs:192
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:68
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:67
    addWindowListener jar:file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/extensions/userChromeJS@kamens.us.xpi!/experiments.js:178
    result resource://gre/modules/ExtensionParent.sys.mjs:1228
    withCallContextData resource://gre/modules/ExtensionParent.sys.mjs:671
    result resource://gre/modules/ExtensionParent.sys.mjs:1227
    withPendingBrowser resource://gre/modules/ExtensionParent.sys.mjs:681
    result resource://gre/modules/ExtensionParent.sys.mjs:1226
    callAndLog resource://gre/modules/ExtensionParent.sys.mjs:1177
    recvAPICall resource://gre/modules/ExtensionParent.sys.mjs:1225
ExtensionSupport.sys.mjs:231:17
    checkAndRunExtensionCode resource:///modules/ExtensionSupport.sys.mjs:231
    _checkAndRunMatchingExtensions resource:///modules/ExtensionSupport.sys.mjs:192
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:68
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:67
    addWindowListener jar:file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/extensions/userChromeJS@kamens.us.xpi!/experiments.js:178
    result resource://gre/modules/ExtensionParent.sys.mjs:1228
    withCallContextData resource://gre/modules/ExtensionParent.sys.mjs:671
    result resource://gre/modules/ExtensionParent.sys.mjs:1227
    withPendingBrowser resource://gre/modules/ExtensionParent.sys.mjs:681
    result resource://gre/modules/ExtensionParent.sys.mjs:1226
    callAndLog resource://gre/modules/ExtensionParent.sys.mjs:1177
    recvAPICall resource://gre/modules/ExtensionParent.sys.mjs:1225

I see two of these, the second with a shorter trace back:

14:19:41.762 SyntaxError: redeclaration of const WINDOW_ID_COMMON_DIALOG
    <anonymous> file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/chrome/userChrome.js:1
    paint jar:file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/extensions/userChromeJS@kamens.us.xpi!/experiments.js:210
    checkAndRunExtensionCode resource:///modules/ExtensionSupport.sys.mjs:221
    _checkAndRunMatchingExtensions resource:///modules/ExtensionSupport.sys.mjs:192
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:68
    registerWindowListener resource:///modules/ExtensionSupport.sys.mjs:67
    addWindowListener jar:file:///C:/Users/markb/AppData/Roaming/thunderbird/Profiles/4x4rl22v.default-release/extensions/userChromeJS@kamens.us.xpi!/experiments.js:178
    result resource://gre/modules/ExtensionParent.sys.mjs:1228
    withCallContextData resource://gre/modules/ExtensionParent.sys.mjs:671
    result resource://gre/modules/ExtensionParent.sys.mjs:1227
    withPendingBrowser resource://gre/modules/ExtensionParent.sys.mjs:681
    result resource://gre/modules/ExtensionParent.sys.mjs:1226
    callAndLog resource://gre/modules/ExtensionParent.sys.mjs:1177
    recvAPICall resource://gre/modules/ExtensionParent.sys.mjs:1225
ExtensionSupport.sys.mjs:231:17

It's almost as if somehow the script is running THREE (???) times in the same context. First time with no error, then 2nd and 3rd time with the redeclaration.

This has me quite confused.