Identical build stage names are suggested multiple times

[rcjsuen]

FROM node AS setup
FROM node AS setup
FROM node AS setup
FROM node AS setup
FROM node AS setup
COPY --from=

Given the above Dockerfile, invoking content assist at the bottom will suggest five build stages all named setup. Having multiple build stages named the same is an error and a diagnostic is being published for this (see #133) but that shouldn't mean that we should "ignore" the error and pollute the list of completion items by having identical entries repeat itself.