Rasterio merging went wrong

I try to merge some multiband rasterfiles (Sentinel 2 data) with Rasterio into a single rasterfile but the result is incorrect.
The files are too big to fit in my RAM completely, but I have enough SWAP and I don’t get a Error Message.
Any ideas, what could have gone wrong?

My code:

import rasterio
from rasterio.merge import merge
import glob

files = glob.glob("../data_multiband/*.tif")
mosaic = []
for file in files:
    tmp = rasterio.open(file)
    mosaic.append(tmp)
mosaic, out_trans = merge(mosaic)
out_meta = tmp.meta.copy()
out_meta.update({"driver": "GTiff",
                 "height": mosaic.shape[1],
                 "width": mosaic.shape[2],
                 "transform": out_trans})
with rasterio.open("../data_merged/test.tif", "w", **out_meta) as dest:
    dest.write(mosaic)

That is how the result looks:

As you can see (red marks) some rasterfiles occur two times. The one on the left side are wrong.
Thats how the original files look:

gdalwarp  -t_srs "EPSG:XXXX" -tr X Y in1.tif in2.tif out.tif