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Your idea is wrong. You should first take out the original value from LocalStorage, then superimpose the new data into the original value, and then store it in LocalStorage again. This way there will be no data overwriting

Do you want to save different item for each tr?

Overwriting is because the name does not change every time you save it
You can change it to this
var nums = 0;
$(".top").click(function(){

            var contrastdata = new Object;
            contrastdata.Machinedata = $(this).parents('tr').find('td')[0].innerText;
            contrastdata.UserNamedata = $(this).parents('tr').find('td')[2].innerText;
            contrastdata.InstrumentIDdata = $(this).parents('tr').find('td')[4].innerText;
            localStorage.setItem('contrastdata_'+nums,JSON.stringify(contrastdata)); 
            nums++;
        }); 

When taking the value below, loop based on the value of nums
or change the data structure, use an object to store data, add the data to the object every time you click, and then store the object in localstroage

First take out the data from localStorage and convert it into an object. Remember to make defensive judgments, then insert the value you want to save into the taken out object, and then convert it into a string and store it in localStorage to overwrite the original data;

Read first, then append, then write.

// 讀取已存入的數(shù)據(jù)；
// `|| []`是為了第一次存取時(shí)，初始化存入的數(shù)據(jù)結(jié)構(gòu)，想要追加就得用數(shù)組
let temp = JSON.parse(localStorage.getItem('contrastdata')) || [];
// 追加數(shù)據(jù)
temp.push({
    Machinedata: 'abc',
    UserNamedata: 123
});
    
localStorage.setItem(temp);

At the beginning, first assign the value to the variable, and then perform the operation in the click event. The data does not need to be stored directly for the time being. It will be stored when the page jumps or needs to be stored. Otherwise, it will be saved every time it is clicked. Once, and then take it out, it is very troublesome.

var contrastdata = JSON.parse(localStorage.getItem('contrastdata'));
if(!contrastdata){
   contrastdata = {};
}
$(".top").click(function(){
     contrastdata.Machinedata = $(this).parents('tr').find('td')[0].innerText;
     contrastdata.UserNamedata = $(this).parents('tr').find('td')[2].innerText;
     contrastdata.InstrumentIDdata = $(this).parents('tr').find('td')[4].innerText;
});

//頁面做跳轉(zhuǎn)或者需要存的時(shí)候再存儲(chǔ)
localStorage.setItem('contrastdata',JSON.stringify(contrastdata));

var contrastdata = JSON.parse(localStorage.getItem('contrastdata')),contrastdata1={},contrastdataArr=[];
//如果localStorage.getItem('contrastdata')存在值，就先添加進(jìn)數(shù)組里面
if(contrastdata){
    contrastdataArr.push(contrastdata)
}
$(".top").click(function(){
    //初始化contrastdata1臨時(shí)變量
     var contrastdata1={};
     //設(shè)置contrastdata1
     contrastdata1.Machinedata = $(this).parents('tr').find('td')[0].innerText;
     contrastdata1.UserNamedata = $(this).parents('tr').find('td')[2].innerText;
     contrastdata1.InstrumentIDdata = $(this).parents('tr').find('td')[4].innerText;
     //把contrastdata1添加進(jìn)contrastdataArr;
     contrastdataArr.push(contrastdata1)
});
//頁面關(guān)閉時(shí)把contrastdataArr存儲(chǔ)進(jìn)localStorage(contrastdata)
localStorage.setItem('contrastdata',JSON.stringify(contrastdataArr));