Netherlands -Luxembourg
We didn't know about this fact that our flags are matching with some others flag and now we have to know who copied first so let's take a look at the first one where the state flag of the Netherlands was first introduced in 1937 whereas the flag of Luxembourg was established in 1830 while their only difference is the shade of the color red and blue.

Australia - New Zealand
Australia and New Zealand have the same flag but they have one slight difference and that is the seven-pointed star of the flag where the one has is white in color whereas the other is in red color.

Ireland - Ivory Coast
The tricolor of both the flag is in vertical form whereas Ireland have green color first whereas Ivory coast has an orange color.

Nigaragua - Honduras - Argentina
They have one thing in common and that is the color of their flag whereas their national flag middle symbol denotes different ideologies.

America - Malaysia
Both of the flags look so similar except for one difference where America has 50 stars and Malaysia's flag represent Islamic religion which is the official religion of the country.

Italy - Mexico
They both have same color triband except the national flag of Mexico has the national coats of arms charged at the center.

Egypt-Iraq-Syria-Sudan-Yemen
All the 5 flags denote the unity of the Arab nations that's why their color looks similar whereas Egypt, Iraq, and Syria have different designs on their center indicating the important things written on their flag center.

Qatar-Bahrain
Qatar and Bahrain have different shades of red where Qatar's flag declared its independence from the British with the color change but the only difference is that Bahrain's flag has only 5 points.

Monaco - Indonesia
They both have the same positions and same color except the part where their shade of color red is little different that we can see them apart.

India - Niger
Both of them have the same color except the flag of niger have different shades of orange and also at the center whereas India's tricolor has 24 stripes in the middle while niger's flag was adopted in 1959 and Indian flag on 1947.