The e-commerce events act globally on all products listed in the products variable, so if you have e.g.
s.events="scRemove,scAdd";
s.products=";sku1,;sku2";
Adobe Analytics will record a scRemove and scAdd event for both products. So, if you have an existing product (sku1) you already previously added (scAdd) and want to remove it (scRemove) and then add (scAdd) another product (sku1), you will need to make two separate AA calls to accomplish this.
Example:
Original scAdd call (for context):
s.events = "scAdd";
s.products = ";sku1";
s.linkTrackVars="scAdd,events,products";
s.linkTrackEvents="scAdd";
s.tl(true,"o","cart add");
First, remove product sku1:
s.events = "scRemove";
s.products = ";sku1";
s.linkTrackVars="scRemove,events,products";
s.linkTrackEvents="scRemove";
s.tl(true,"o","cart remove");
Then, add the new product sku2:
s.events = "scAdd";
s.products = ";sku2";
s.linkTrackVars="scAdd,events,products";
s.linkTrackEvents="scAdd";
s.tl(true,"o","cart add");