Why do 2 copies of Zotero launch at the same time?

Why do two copies of Zotero launch (and appear in the dock) after I click (only once) the Zotero icon in Launchpad?
This means that 2 copies of Zotero are running simultaneously in my MacBook Pro.
As a result, I cannot install Zotero's latest update.
Please advise. Thanks!
Zotero 5.0.37
macOS High Sierra 10.13.5
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