Basically the crux of the argument on whether an ability could (or couldn't) be stacked is whether it is either a triggered ability or a replacement ability. While both of these two looks for a condition for the effect to be fulfilled, there is a strict definition on replacement effect (CR709.1).
For triggered effects, as long as two effects with the same name affecting a character is resolved in a wave. Only one of the effect is resolved on that character. However, replacement effects are not resolved in waves like triggered effects. When multiple replacement effects can replace a certain event, the character who is affected chooses one replacement effect to replace the event first and subsequently chooses another (if another still exists to replace the modified event).
Hopefully this clears the issue up a little.
CR1.05:
709.1 Effects that use both “would” and “instead” are replacement effects. A replacement effect replaces a specified event with one or more modified events. The specified event never happens, so no abilities can trigger off it. Instead, the modified events happen, which may in turn trigger abilities.
Some people learn, some people don't. For me, I can never correct myself to say Julia Graves as opposed to Julia Roberts