JSFiddle - React, Tailwind, and code Playground

JavaScript

function countAdjacentPairs(searchString) {
  var items = searchString.toLowerCase().split(' ').sort();
  var matchedItems = [];

  for(i=0; i<items.length; i++) {
    if(items[i] === items[(i+1)]
      && matchedItems.indexOf(items[i]) === -1) {
      matchedItems.push(items[i]);
    }
  }
  return matchedItems.length;
}

document.write(countAdjacentPairs('cat dog cat dog cat dog'));