You can ease the code by always retrieving the List value in the for loop, then if it is null create a new one and add it to your Map, otherwise add the value to the list.
for (int i = 0; i < records.length; i++) { List<String> value = sfFundIdsByContact.get(breakdown.getSalesConnect__Contact__c()); if (value == null) { value = new ArrayList<String>(); sfFundIdsByContact.put(breakdown.getSalesConnect__Contact__c(), value); } value.add(breakdown.getId()); } As a recommendation, change the definition of
LinkedHashMap<String, ArrayList<String>> sfFundIdsByContact to
Map<String, List<String>> sfFundIdsByContact