These two words are the same in meanings, but they are different in grammar. 看た (みた) is a simple past 見かけて (みかけて) is a verb-gerund

"Mita" -dictionary form is "miru" It means to watch, see(but not meet), take a look, gaze, witness, etc. You can "miru" intentionally or not. "Mikaketa"- dictionary form is "mikakeru" The meaning is much narrower than "miru". You can only "mikakeru" unintentionally. If you "mikaketa" someone somewhere, you had not expected to see him there, and you didn't greet him there. You just witnessed his being there accidentally.

From the Japanese Dictionary 【みた】 <Verb - Past Plain>; (v1,vt) to look after (often medically); to take care of; 【みかけて】 <Verb - Gerund>; (v1,vt) to (happen to) see; to notice; to catch sight of; (P).