Using stream you could do:

ArrayList<HashMap<String, Double>> listHashMaps = // your list of maps

Map<String,Double> mergedMap = 
        listHashMaps.stream()
                .flatMap(m -> m.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Double::sum));

One option is to use the Map.compute method. Its second argument is a remapping function that can be used like in the following example:

Map<String, Double> inputMap1 = Map.of("X1", 5.0, "X2", 3.0);
Map<String, Double> inputMap2 = Map.of("X1", 7.0, "X3", 9.0);
List<Map<String, Double>> listOfMaps = List.of(inputMap1, inputMap2);

Map<String, Double> mergedMap = new HashMap<>();
for (Map<String, Double> map : listOfMaps) {
    map.forEach((key, value) -> {
        mergedMap.compute(key, (k, oldValue) -> oldValue == null ? value : oldValue + value);
    });
}

System.out.println(mergedMap);

oldValue is null if there is not yet an entry with key in mergedMap, and the old value otherwise.

This prints:

{X1=12.0, X2=3.0, X3=9.0}

Update: I might prefer the concise solution by Alexander Ivanchenko. I will leave this Answer posted as an interesting or educational alternative.

Your example data is flawed (repeated keys), so I crafted another set of data.

Map < String, Double > map1 =
        Map.of(
                "X1" , 1d ,
                "X2" , 1d
        );

Map < String, Double > map2 =
        Map.of(
                "X1" , 2d ,
                "X2" , 2d ,
                "X3" , 7d
        );

Define a new Map to hold results.

Map < String, Double > results = new HashMap <>();

Stream & Map#getOrDefault

Process the two input maps by making a stream of each, joining those streams into a single stream. For each map entry in that stream, put the key into the new map, with a value of either the entry's value upon first occurrence or adding the entry's value to the previously put value.

Stream
        .concat( map1.entrySet().stream() , map2.entrySet().stream() )
        .forEach(
                stringDoubleEntry ->
                        results.put(
                                stringDoubleEntry.getKey() ,                                                                        // key
                                results.getOrDefault( stringDoubleEntry.getKey() , 0d ) + stringDoubleEntry.getValue() )  // value
        );

System.out.println( "results.toString() = " + results );

See this code run live at Ideone.com.

map = {X1=3.0, X2=3.0, X3=7.0}

Without streams

If not yet familiar with streams, you could instead use a pair of for-each loops to process each of the two input maps.

Process the first map.

for ( Map.Entry < String, Double > stringDoubleEntry : map1.entrySet() )
{
    String k = stringDoubleEntry.getKey();
    Double v =
            results
                    .getOrDefault( stringDoubleEntry.getKey() , 0d )  // Returns either (A) a value already put into `results` map, or else (B) a default value of zero.
                    + stringDoubleEntry.getValue();
    results.put( k , v );  // Replaces any existing entry (key-value pair) with this pair.
}

And do the same for the second input map. The only difference here is the first line, map1.entrySet() becomes map2.entrySet().

for ( Map.Entry < String, Double > stringDoubleEntry : map2.entrySet() )
{
    String k = stringDoubleEntry.getKey();
    Double v =
            results
                    .getOrDefault( stringDoubleEntry.getKey() , 0d )  // Returns either (A) a value already put into `results` map, or else (B) a default value of zero.
                    + stringDoubleEntry.getValue();
    results.put( k , v );  // Replaces any existing entry (key-value pair) with this pair.
}

To merge two maps by keeping the original entries, while summing the values with same keys, you could:

1. concat() the streams of the maps' entries.
2. Collect with the overloaded method Collectors.toMap() to map each value with their original key, while combining values with colliding keys with a merge function (in your case, the method reference Double::sum).

Stream.concat(map1.entrySet().stream(), map2.entrySet().stream())
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Double::sum));

Here is a "You can't teach and old dog new tricks" way of doing it:

public static Map<String, Double> addMapValues 
        (Collection<Map<String, Double>> mapsIn) {
    Map<String, Double> sums = new HashMap<> ();   
    
    for (Map<String, Double> aMap : mapsIn) {
        for (Map.Entry<String, Double> entry: aMap.entrySet()) {
            Double runningTotal = sums.get(entry.getKey());
            if (runningTotal == null) {
                sums.put(entry.getKey(), entry.getValue());
            } else {
                runningTotal += entry.getValue();
                sums.put(entry.getKey(), runningTotal);                    
            }
        }                
    }
    return sums;
}