Doubts separating tree entries into different trees

ROOT
3

Mmh no, that’s not what I have here. Let’s take this updated version:

void do_ROOT_job(bool upside_down, int ch, double x0, double y0, double r, double l) {

    TFile inFile("../../gedet/surf/ver/ver-gedet-surf-0.root");
    TFile nFile("../../gedet/nplus/ver/ver-gedet-nplus-0.root", "RECREATE");
    TFile pFile("../../gedet/pplus/ver/ver-gedet-pplus-0.root", "RECREATE");

    auto tree = dynamic_cast<TTree*>(inFile.Get("GSSTree"));
    auto pTree = tree->CloneTree(0);
    auto nTree = tree->CloneTree(0);

    double x, y, z;
    tree->SetBranchAddress("x_cm", &x);
    tree->SetBranchAddress("y_cm", &y);
    tree->SetBranchAddress("z_cm", &z);

    int N = tree->GetEntries();
    if (upside_down) {
        for (int i = 0; i < N; ++i) {
            tree->GetEntry(i);
            if ( (x0-x)*(x0-x)+(y0-y)*(y0-y) <= r*r and z > l ) pTree->Fill();
            else                                                nTree->Fill();
        }
    }

    else {
        for (int i = 0; i < N; ++i) {
            tree->GetEntry(i);
            if ( (x0-x)*(x0-x)+(y0-y)*(y0-y) <= r*r and z < l ) pTree->Fill();
            else                                                nTree->Fill();
        }
    }

    pFile.WriteTObject(pTree);
    nFile.WriteTObject(nTree);
}

Now I’m trying to save the two cloned trees into different files. The result is the following:

$ root ver-gedet-pplus-0.root
root [0]
Attaching file ver-gedet-pplus-0.root as _file0...
(TFile *) 0x7fa96fdb1ef0
root [1] .ls
TFile**		ver-gedet-pplus-0.root
 TFile*		ver-gedet-pplus-0.root
  KEY: TTree	GSSTree;3	General Surface Sampler Coordinate Tree
  KEY: TTree	GSSTree;2	General Surface Sampler Coordinate Tree

$ root ver-gedet-nplus-0.root
root [0]
Attaching file ver-gedet-nplus-0.root as _file0...
(TFile *) 0x7f98a90e3430
root [1] .ls
TFile**		ver-gedet-nplus-0.root
 TFile*		ver-gedet-nplus-0.root
  KEY: TTree	GSSTree;1	General Surface Sampler Coordinate Tree

The doubling of trees in the first file is actually what I don’t understand.