@[email protected]'s Solution works wonderfully.

Here is another way using the exclude-items-plugin:

exclude = {import:exclude-items-plugin}

sentence
  [takenTypes = [], ''] You find yourself in {a} [d=description.selectOne] [b=biome.selectOne].<br> As the area is [d.singularForm.lowerCase], [td = typesdescription.selectOne.evaluateItem, takenTypes.push(td), td] type 'mons are rather prolific. That is in addition to the [typesbiome = exclude(others[b], takenTypes).selectUnique(2).map(a => (takenTypes.push(a), a)).joinItems(" and ")] types that live in the [b] already. <br>Oddly enough, there also seem to be some [s=exclude(stragglers, takenTypes).selectOne, takenTypes.push(s), s] type 'mons in the area as well. <br><br> Pokemon Types: [td] ([d]), [typesbiome] ([b]), [s] (stragglers)

The td = typesdescription.selectOne.evaluateItem makes sure that you have the Type instead of the description stored on the takenTypes list. If you don't want to add the evaluateItem you could remove the dynamic odds using the d and use td = typesdescription[d].selectOne to select only the type. We push the selected types on the takenTypes list and use it to exclude the already selected types on the next selection with the exclude-items-plugin.

Here's my solution so far - create a variable that'll hold the generated types in and then change the code of the sentence list to never use any of these already-generated types:

// Create a new "takenTypes" array variable to store the already generated types
takenTypes = [[]]

sentence
  [takenTypes = [], ""]You find yourself in {a} [d=description.selectOne] [b=biome.selectOne].<br> As the area is [d.singularForm.lowerCase], [td = typesdescription.selectOne, uniqueReturn(td)] type 'mons are rather prolific. That is in addition to the [removeDups(others[b]).selectUnique(2).map(x => uniqueReturn(x)).join(" and ")] types that live in the [b] already. <br>Oddly enough, there also seem to be some [s=removeDups(stragglers).selectOne, uniqueReturn(s)] type 'mons in the area as well.

// Returns a new list that does not contain the entries already shown in the output
removeDups(list) => return list.selectAll.filter(x => !takenTypes.includes(x));
// Adds the entry into the "takenTypes" variable so they never show up again in the same output
uniqueReturn(i) => takenTypes.push(i); return i;

Does selectUnique and consumableList not cover you?