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Compile error · C++

Fix “redefinition of 'void setup()'” in Arduino

Your sketch has two setup() (or loop()) functions — two pasted examples or a second .ino tab. Merge them into one setup() and one loop().

All boardsUpdated 3 Nov 2026

Community draft — report a mistake

What you see

OutputOutput
WeatherStation.ino: In function 'void setup()':
WeatherStation.ino:9:6: error: redefinition of 'void setup()'
 void setup() {
      ^~~~~
WeatherStation.ino:1:6: note: 'void setup()' previously defined here
 void setup() {
      ^~~~~
exit status 1

Compilation error: redefinition of 'void setup()'

Every Arduino sketch has exactly one setup() and one loop(). When you combine two examples, you get two of each, and the compiler stops at the second definition.

// One setup() that does both jobs
void setup() {
  Serial.begin(115200);
  dht.begin();          // from the sensor example
  lcd.init();           // from the display example
  lcd.backlight();
}

Quick fix — try in order

  1. 1

    Merge the two setup() functions

    Copy the lines from inside the second setup() { … } into the first, then delete the second one completely. Do the same for loop().

  2. 2

    Check other tabs

    All .ino tabs in a sketch folder are joined into one file before compiling. A second tab with its own setup() — often a pasted example — causes this error. Delete or rename it to .txt.

  3. 3

    Check the sketch folder

    An old Copy_of_sketch.ino sitting in the same folder is compiled too. Move it out.

  4. 4

    Merge globals and includes too

    Each example also has its own #include lines and global objects; keep one of each and rename clashes (e.g. two display objects).

Why it happens

Combining two examples

The classic case: a DHT example and an LCD example pasted into one file. Each brings its own setup() and loop(). A sketch has exactly one of each.

redefinition of a variable or object

redefinition of 'DHT dht' — the same global declared twice. Keep one.

multiple definition vs redefinition

Redefinition is caught by the compiler inside one file (all .ino tabs count as one). Duplicates across .cpp files show up later as a linker error — see multiple definition of.

Board-specific notes

Arduino IDE 2: Tabs are shown along the top of the editor. The folder name must match the main .ino name.

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