Praming Puzles & Colf: Condense a String
JavaScript (ES6), 74 bytes



f=
(s,n=0,m=s.match(`(.).{${n}}\\1`))=&gt;s[n]?m?f(s.replace(...m)):f(s,n+1):s
;
&lt;input oninput=o.textContent=f(this.value)&gt;&lt;pre id=o&gt;