How to use if then in google app scripts?

I'm working on an app that scans data into Google sheets. i have two sheets: sheet1 and sheet2. In sheet1 the data is entered into it after a scan is performed. But before the scan data can be entered, i would like to do an if function to check if there is a similar type of data in sheet2 before entering the data. if there isn't one then a message will be displayed such as "There is no such person in the database". I would also like to specify the column to check for similarity in sheet2 e.g A2:A100 Below is the script code

function doGet(e){
var ss = SpreadsheetApp.openByUrl("google sheet url");
//Give your Sheet name here
var sheet = ss.getSheetByName("Sheet1");

return insert(e,sheet);

}

function doPost(e){
var ss = SpreadsheetApp.openByUrl("google sheet url");
//Give your Sheet name here
var sheet = ss.getSheetByName("Sheet1");

return insert(e,sheet);

}



function insert(e,sheet){

 // reciving scanned data from client i.e android app
 var scannedData = e.parameter.sdata;
 var d = new Date();
 var ctime= d.toLocaleString();

 sheet.appendRow([scannedData,ctime]);

 return ContentService
.createTextOutput("Success")
.setMimeType(ContentService.MimeType.JAVASCRIPT);  


}

Thanks.