Fix issue of duplicated blocks upon pasting an image into the Slate E… #15848

Job	Run time
Core Basic (18.x)	6m 41s
Core Content (18.x)	2m 51s
Core Basic (20.x)	5m 50s
Core Content (20.x)	2m 41s
Core Control Panels (18.x)	3m 40s
Core Control Panels (20.x)	3m 44s
Core Basic - Plone 5 (18.x)	6m 41s
Core Blocks (18.x)	6m 57s
Core Blocks - Listing (18.x)	4m 27s
Core Blocks (20.x)	7m 9s
Core Blocks - Listing (20.x)	4m 0s
Coresandbox (18.x)	4m 30s
Core Volto Slate (18.x)	6m 38s
Project Generator (18.x)	6m 40s
Project Generator (20.x)	6m 22s
Multilingual (18.x)	2m 54s
Core Volto Slate (20.x)	6m 45s
Working Copy (18.x)	2m 56s
Multilingual in Seamless Mode (18.x)	2m 54s
Seamless Mode (18.x)	6m 13s
	1h 40m 33s

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