Why Firebase .onValue not working properly - flutter

Question

I have been trying to make an app that represent a graph from values in firebase. I am using onValue.listen to get values when changed. I don't know what exactly happens, but when I change value manually, the same value seems to be repeated multiple times, so not one value is added but multiple in one single event. It is like if the function has been called more than once. I am using syncfusion_flutter_charts to build the graph.
Code for listening:
void getCurrent() {
currentRef.onValue.listen((event) {
if (mounted) {
double currentReading = double.parse(event.snapshot.value.toString());
int listLength = currentChartData.length;
print(currentChartData);
setState(() {
currentChartData
.add(CurrentChartData(listLength, currentReading.round()));
});
}
});
}
code for graph:
SfCartesianChart(
zoomPanBehavior: ZoomPanBehavior(
zoomMode: ZoomMode.x,
maximumZoomLevel: 0.04 * currentChartData.length,
enablePinching: true,
enablePanning: true,
enableSelectionZooming: true),
borderColor: Colors.transparent,
primaryXAxis: NumericAxis(
minimum: 1,
rangePadding: ChartRangePadding.round,
interval: 1.0),
primaryYAxis: NumericAxis(
minimum: 0,
rangePadding: ChartRangePadding.round,
),
series: <ChartSeries<CurrentChartData, int>>[
SplineAreaSeries<CurrentChartData, int>(
dataSource: currentChartData,
xValueMapper: (CurrentChartData data, _) => data.day,
yValueMapper: (CurrentChartData data, _) => data.ppm,
name: 'Methane Concentrations',
gradient: LinearGradient(
colors: [
AppStyle.spline_color,
AppStyle.bg_color.withAlpha(150)
],
begin: Alignment.topCenter,
end: Alignment.bottomCenter,
),
),
SplineSeries(
dataSource: currentChartData,
xValueMapper: (CurrentChartData data, _) => data.day,
yValueMapper: (CurrentChartData data, _) => data.ppm,
color: AppStyle.accent_color,
width: 3,
)
],
),