Preventing double borders in CSS Grid

Solution 1:

Instead of using an actual border around grid items, use the background color on the container (for "border" color) and the grid-gap property (for "border" width).

.wrapper {
  display: inline-grid;
  grid-template-columns: 50px 50px 50px 50px;
  border: 1px solid black;
  grid-gap: 1px;
  background-color: black;
}

.wrapper > div {
  background-color: white;
  padding: 15px;
  text-align: center;
}
<div class="wrapper">
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
  <div>5</div>
  <div>6</div>
  <div>7</div>
  <div>8</div>
</div>

Solution 2:

You may do like this :

.wrapper {
  display: inline-grid;
  grid-template-columns: 50px 50px 50px 50px;
  border-bottom: 1px solid black;
  border-left: 1px solid black;
}

.wrapper > div {
  padding: 15px;
  text-align: center;
  border-top: 1px solid black;
  border-right: 1px solid black;
}

body {
 background:pink;
}
<div class="wrapper">
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
  <div>5</div>
  <div>6</div>
  <div>7</div>
  <div>8</div>
</div>

Another idea is to rely on gradient to fill gaps like below:

.wrapper {
  display: inline-grid;
  grid-template-columns: 50px 50px 50px 50px;
  grid-gap:1px;
  background:
    linear-gradient(#000,#000) center/100% 1px no-repeat,
    repeating-linear-gradient(to right,transparent 0 50px,#000 0 51px);
  border:1px solid;
}

.wrapper > div {
  padding: 15px;
  text-align: center;
}

body {
 background:pink;
}
<div class="wrapper">
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
  <div>5</div>
  <div>6</div>
  <div>7</div>
  <div>8</div>
</div>

You can also adjust the initial solution to make it more flexible and it will work with any number of items inside a row.

Run the below code on full page and resize the window:

.wrapper {
  display: grid;
  max-width:800px;
  grid-template-columns: repeat(auto-fill,minmax(100px,1fr));
  border-top: 1px solid black;
  border-left: 1px solid black;
}

.wrapper > div {
  padding: 15px;
  text-align: center;
  border-bottom: 1px solid black;
  border-right: 1px solid black;
}

body {
 background:pink;
}
<div class="wrapper">
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
  <div>5</div>
  <div>6</div>
  <div>7</div>
  <div>8</div>
  <div>9</div>
  <div>10</div>
  <div>11</div>
</div>