esriEditing polygons and polygon boundaries can be tricky and often lead to unwanted duplication or error. These instructions can help guide the user in choosing the correct tool for the job.

WHEN AN EXTERNAL POLYGON IS PRESENT:
To add polygons that have duplicate boundaries use the “cut polygon features” under “task”.
When adding a new internal polygon using pre-existing internal linework:
(1) Select the internal polygon
(2) Editor –> clip
(3) Select the external polygon
(4) Task –> cut polygon features
(5) Draw the new polygon
WHEN NO EXTERNAL POLYGON IS PRESENT:
When connecting multiple island polygons with a single polygon:
(1) Task –> auto-complete polygon
(2) Start inside the polygon or snap to the edge
(3) When you get to the pre-existing linework or inside the polygon do a left-click
(4) Right click –> finish part
(5) Do this until the end, at the end do finish sketch
Creating doughnut holes in polygons in ArcMap:
(1) Start editing the layer
(2) Select the internal polygon
(3) Go to Editor –> Clip
(4) Choose ‘Discard the area that intersects’
(5) Now if you want a polygon with a hole delete the small (internal) polygon.