Use this script to efficiently retrieve the addresses you want (given the time span) and remove duplicates:
Code
function getEmails() {
// http://stackoverflow.com/a/12029701/1536038
// get all messages
var eMails = GmailApp.getMessagesForThreads(
GmailApp.search('after:2012/1/14 before:2013/8/15'))
.reduce(function(a, b) {return a.concat(b);})
.map(function(eMails) {
return eMails.getFrom()
});
// sort and filter for unique entries
var aEmails = eMails.sort().filter(function(el,j,a)
{if(j==a.indexOf(el))return 1;return 0});
// create 2D-array
var aUnique = new Array();
for(var k in aEmails) {
aUnique.push([aEmails[k]]);
}
// add data to sheet
SpreadsheetApp.getActiveSheet().getRange(1, 1, aUnique.length, 1)
.setValues(aUnique);
}