Stop multiple appearances of a widget in the same window

[Akame.L]

Hello,
I have a vertical layout (called center) & 3 buttons (aide,info,prom).
for each button clicked i want to add a specific widget into the vertical layout.
the problem is everytime a button is clicked it specific wigdet will apear if i click another botton or the same button again it will add another wigdet next to the previous one.
How can I stop that from happening.
Here what I have so far:

...
void employe::on_aide_clicked()
{
    aide *a = new aide();
    ui->center->addWidget(a,Qt::AlignLeft);
}
void employe::on_info_clicked()
{
    empInfo *i = new empInfo();
    ui->center->addWidget(i,Qt::AlignLeft);
}
 
void employe::on_prom_clicked()
{
    demPro *d = new demPro();
    ui->center->addWidget(d,Qt::AlignLeft);
}

...
void employe::on_aide_clicked()
{
    aide *a = new aide();
    ui->center->addWidget(a,Qt::AlignLeft);
}
void employe::on_info_clicked()
{
    empInfo *i = new empInfo();
    ui->center->addWidget(i,Qt::AlignLeft);
}

void employe::on_prom_clicked()
{
    demPro *d = new demPro();
    ui->center->addWidget(d,Qt::AlignLeft);
}

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